cpu/stm32_common: fix DMA releasing in UART driver
cpu/stm32_common: fix DMA releasing in UART driver

basilfx/RIOT,smlng/RIOT,mfrey/RIOT,mtausig/RIOT,smlng/RIOT,miri64/RIOT,toonst/RIOT,mtausig/RIOT,authmillenon/RIOT,x3ro/RIOT,mfrey/RIOT,mfrey/RIOT,OTAkeys/RIOT,kaspar030/RIOT,jasonatran/RIOT,aeneby/RIOT,basilfx/RIOT,aeneby/RIOT,OlegHahm/RIOT,kaspar030/RIOT,x3ro/RIOT,yogo1212/RIOT,authmillenon/RIOT,ant9000/RIOT,josephnoir/RIOT,ant9000/RIOT,OlegHahm/RIOT,basilfx/RIOT,kYc0o/RIOT,cladmi/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,kaspar030/RIOT,aeneby/RIOT,toonst/RIOT,yogo1212/RIOT,toonst/RIOT,OlegHahm/RIOT,smlng/RIOT,OlegHahm/RIOT,authmillenon/RIOT,yogo1212/RIOT,authmillenon/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,rfuentess/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,cladmi/RIOT,RIOT-OS/RIOT,aeneby/RIOT,mfrey/RIOT,ant9000/RIOT,lazytech-org/RIOT,yogo1212/RIOT,kYc0o/RIOT,rfuentess/RIOT,yogo1212/RIOT,josephnoir/RIOT,OTAkeys/RIOT,miri64/RIOT,aeneby/RIOT,cladmi/RIOT,jasonatran/RIOT,kYc0o/RIOT,ant9000/RIOT,jasonatran/RIOT,authmillenon/RIOT,jasonatran/RIOT,cladmi/RIOT,jasonatran/RIOT,x3ro/RIOT,x3ro/RIOT,mfrey/RIOT,toonst/RIOT,miri64/RIOT,kYc0o/RIOT,smlng/RIOT,kaspar030/RIOT,mtausig/RIOT,josephnoir/RIOT,OlegHahm/RIOT,basilfx/RIOT,authmillenon/RIOT,lazytech-org/RIOT,mtausig/RIOT,lazytech-org/RIOT,smlng/RIOT,lazytech-org/RIOT,rfuentess/RIOT,rfuentess/RIOT,miri64/RIOT,x3ro/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,toonst/RIOT,cladmi/RIOT,lazytech-org/RIOT,basilfx/RIOT,mtausig/RIOT,ant9000/RIOT,josephnoir/RIOT,rfuentess/RIOT,OTAkeys/RIOT