cpu/cc2538: clear only the latched UART interrupt flags
cpu/cc2538: clear only the latched UART interrupt flags

Otherwise incoming bytes can remain in the receive FIFO indefinitely.

adrianghc/RIOT,Josar/RIOT,immesys/RiSyn,shady33/RIOT,A-Paul/RIOT,dkm/RIOT,kbumsik/RIOT,kaspar030/RIOT,yogo1212/RIOT,shady33/RIOT,roberthartung/RIOT,zhuoshuguo/RIOT,LudwigKnuepfer/RIOT,lazytech-org/RIOT,kaspar030/RIOT,asanka-code/RIOT,thomaseichinger/RIOT,jfischer-phytec-iot/RIOT,x3ro/RIOT,malosek/RIOT,plushvoxel/RIOT,backenklee/RIOT,smlng/RIOT,aeneby/RIOT,Hyungsin/RIOT-OS,Josar/RIOT,jbeyerstedt/RIOT-OTA-update,A-Paul/RIOT,gebart/RIOT,x3ro/RIOT,rajma996/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,neumodisch/RIOT,beurdouche/RIOT,josephnoir/RIOT,mfrey/RIOT,x3ro/RIOT,thomaseichinger/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,roberthartung/RIOT,kYc0o/RIOT,mtausig/RIOT,jremmert-phytec-iot/RIOT,asanka-code/RIOT,altairpearl/RIOT,miri64/RIOT,asanka-code/RIOT,MohmadAyman/RIOT,cladmi/RIOT,adrianghc/RIOT,basilfx/RIOT,jbeyerstedt/RIOT-OTA-update,FrancescoErmini/RIOT,adjih/RIOT,FrancescoErmini/RIOT,MohmadAyman/RIOT,kerneltask/RIOT,altairpearl/RIOT,lazytech-org/RIOT,dkm/RIOT,aeneby/RIOT,kYc0o/RIOT,syin2/RIOT,rajma996/RIOT,asanka-code/RIOT,immesys/RiSyn,rousselk/RIOT,alignan/RIOT,FrancescoErmini/RIOT,rfuentess/RIOT,wentaoshang/RIOT,smlng/RIOT,gautric/RIOT,TobiasFredersdorf/RIOT,kerneltask/RIOT,kaspar030/RIOT,A-Paul/RIOT,toonst/RIOT,OTAkeys/RIOT,aeneby/RIOT,biboc/RIOT,hamilton-mote/RIOT-OS,kaleb-himes/RIOT,backenklee/RIOT,malosek/RIOT,backenklee/RIOT,d00616/RIOT,BytesGalore/RIOT,cladmi/RIOT,brettswann/RIOT,neumodisch/RIOT,ant9000/RIOT,jfischer-phytec-iot/RIOT,kYc0o/RIOT,A-Paul/RIOT,syin2/RIOT,rakendrathapa/RIOT,backenklee/RIOT,neumodisch/RIOT,yogo1212/RIOT,smlng/RIOT,dailab/RIOT,dkm/RIOT,josephnoir/RIOT,tfar/RIOT,gautric/RIOT,kaleb-himes/RIOT,Ell-i/RIOT,asanka-code/RIOT,beurdouche/RIOT,jasonatran/RIOT,rousselk/RIOT,gautric/RIOT,altairpearl/RIOT,immesys/RiSyn,asanka-code/RIOT,mfrey/RIOT,neumodisch/RIOT,immesys/RiSyn,kerneltask/RIOT,Hyungsin/RIOT-OS,foss-for-synopsys-dwc-arc-processors/RIOT,jremmert-phytec-iot/RIOT,neiljay/RIOT,A-Paul/RIOT,x3ro/RIOT,josephnoir/RIOT,TobiasFredersdorf/RIOT,miri64/RIOT,smlng/RIOT,ks156/RIOT,zhuoshuguo/RIOT,MohmadAyman/RIOT,BytesGalore/RIOT,rfuentess/RIOT,jfischer-phytec-iot/RIOT,kYc0o/RIOT,hamilton-mote/RIOT-OS,rajma996/RIOT,authmillenon/RIOT,d00616/RIOT,wentaoshang/RIOT,roberthartung/RIOT,Josar/RIOT,gebart/RIOT,katezilla/RIOT,OTAkeys/RIOT,toonst/RIOT,basilfx/RIOT,avmelnikoff/RIOT,kb2ma/RIOT,yogo1212/RIOT,dailab/RIOT,kbumsik/RIOT,mtausig/RIOT,Hyungsin/RIOT-OS,tfar/RIOT,biboc/RIOT,attdona/RIOT,jremmert-phytec-iot/RIOT,authmillenon/RIOT,kerneltask/RIOT,kb2ma/RIOT,roberthartung/RIOT,syin2/RIOT,yogo1212/RIOT,tfar/RIOT,neumodisch/RIOT,kaspar030/RIOT,alignan/RIOT,kbumsik/RIOT,beurdouche/RIOT,immesys/RiSyn,rousselk/RIOT,FrancescoErmini/RIOT,OTAkeys/RIOT,TobiasFredersdorf/RIOT,wentaoshang/RIOT,Ell-i/RIOT,rfuentess/RIOT,avmelnikoff/RIOT,brettswann/RIOT,LudwigOrtmann/RIOT,gebart/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,cladmi/RIOT,d00616/RIOT,kbumsik/RIOT,authmillenon/RIOT,yogo1212/RIOT,rakendrathapa/RIOT,gebart/RIOT,alignan/RIOT,altairpearl/RIOT,zhuoshuguo/RIOT,kerneltask/RIOT,thomaseichinger/RIOT,thomaseichinger/RIOT,jremmert-phytec-iot/RIOT,attdona/RIOT,jremmert-phytec-iot/RIOT,cladmi/RIOT,authmillenon/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,hamilton-mote/RIOT-OS,mtausig/RIOT,dailab/RIOT,biboc/RIOT,attdona/RIOT,wentaoshang/RIOT,josephnoir/RIOT,neiljay/RIOT,kbumsik/RIOT,gautric/RIOT,shady33/RIOT,adrianghc/RIOT,kb2ma/RIOT,OlegHahm/RIOT,backenklee/RIOT,jbeyerstedt/RIOT-OTA-update,malosek/RIOT,josephnoir/RIOT,rakendrathapa/RIOT,kaleb-himes/RIOT,authmillenon/RIOT,Josar/RIOT,Hyungsin/RIOT-OS,tfar/RIOT,smlng/RIOT,jasonatran/RIOT,basilfx/RIOT,neiljay/RIOT,neumodisch/RIOT,FrancescoErmini/RIOT,syin2/RIOT,malosek/RIOT,plushvoxel/RIOT,Ell-i/RIOT,biboc/RIOT,jbeyerstedt/RIOT-OTA-update,kaspar030/RIOT,OlegHahm/RIOT,LudwigOrtmann/RIOT,Ell-i/RIOT,dkm/RIOT,ks156/RIOT,RIOT-OS/RIOT,rajma996/RIOT,jfischer-phytec-iot/RIOT,jremmert-phytec-iot/RIOT,wentaoshang/RIOT,shady33/RIOT,OTAkeys/RIOT,kb2ma/RIOT,alignan/RIOT,kaleb-himes/RIOT,MohmadAyman/RIOT,toonst/RIOT,BytesGalore/RIOT,adjih/RIOT,jasonatran/RIOT,rakendrathapa/RIOT,LudwigOrtmann/RIOT,wentaoshang/RIOT,shady33/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,tfar/RIOT,brettswann/RIOT,mfrey/RIOT,biboc/RIOT,LudwigKnuepfer/RIOT,dailab/RIOT,malosek/RIOT,adjih/RIOT,avmelnikoff/RIOT,miri64/RIOT,ant9000/RIOT,gautric/RIOT,attdona/RIOT,adrianghc/RIOT,kaleb-himes/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,alignan/RIOT,altairpearl/RIOT,TobiasFredersdorf/RIOT,brettswann/RIOT,neiljay/RIOT,katezilla/RIOT,avmelnikoff/RIOT,neiljay/RIOT,rousselk/RIOT,brettswann/RIOT,hamilton-mote/RIOT-OS,d00616/RIOT,miri64/RIOT,rakendrathapa/RIOT,OlegHahm/RIOT,thomaseichinger/RIOT,ant9000/RIOT,attdona/RIOT,authmillenon/RIOT,ant9000/RIOT,jfischer-phytec-iot/RIOT,LudwigOrtmann/RIOT,aeneby/RIOT,TobiasFredersdorf/RIOT,jbeyerstedt/RIOT-OTA-update,plushvoxel/RIOT,plushvoxel/RIOT,BytesGalore/RIOT,rajma996/RIOT,rakendrathapa/RIOT,katezilla/RIOT,rfuentess/RIOT,mtausig/RIOT,rousselk/RIOT,jasonatran/RIOT,LudwigKnuepfer/RIOT,gebart/RIOT,malosek/RIOT,ant9000/RIOT,katezilla/RIOT,yogo1212/RIOT,plushvoxel/RIOT,mfrey/RIOT,dailab/RIOT,beurdouche/RIOT,adjih/RIOT,kb2ma/RIOT,cladmi/RIOT,hamilton-mote/RIOT-OS,brettswann/RIOT,LudwigOrtmann/RIOT,LudwigOrtmann/RIOT,rousselk/RIOT,lazytech-org/RIOT,attdona/RIOT,beurdouche/RIOT,kYc0o/RIOT,mtausig/RIOT,OTAkeys/RIOT,toonst/RIOT,altairpearl/RIOT,RIOT-OS/RIOT,mfrey/RIOT,RIOT-OS/RIOT,adjih/RIOT,LudwigKnuepfer/RIOT,OlegHahm/RIOT,immesys/RiSyn,adrianghc/RIOT,basilfx/RIOT,Josar/RIOT,aeneby/RIOT,d00616/RIOT,BytesGalore/RIOT,toonst/RIOT,dkm/RIOT,basilfx/RIOT,zhuoshuguo/RIOT,rajma996/RIOT,jasonatran/RIOT,roberthartung/RIOT,FrancescoErmini/RIOT,syin2/RIOT,LudwigKnuepfer/RIOT,MohmadAyman/RIOT,zhuoshuguo/RIOT,d00616/RIOT,lazytech-org/RIOT,Hyungsin/RIOT-OS,ks156/RIOT,ks156/RIOT,x3ro/RIOT,shady33/RIOT,miri64/RIOT,ks156/RIOT,lazytech-org/RIOT,MohmadAyman/RIOT,Ell-i/RIOT,katezilla/RIOT,zhuoshuguo/RIOT,avmelnikoff/RIOT