tests/periph_uart: fixed signedness issues
tests/periph_uart: fixed signedness issues

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