kinetis: uart: fix unused-function warning
kinetis: uart: fix unused-function warning

When compiling with LLVM (should also be seen `-Wunused-function` in
GCC), I get an error for `frdm-k22f`. This should fix that.

jasonatran/RIOT,avmelnikoff/RIOT,x3ro/RIOT,toonst/RIOT,yogo1212/RIOT,gebart/RIOT,basilfx/RIOT,mfrey/RIOT,mtausig/RIOT,OTAkeys/RIOT,BytesGalore/RIOT,ant9000/RIOT,miri64/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kYc0o/RIOT,rfuentess/RIOT,yogo1212/RIOT,avmelnikoff/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,jasonatran/RIOT,josephnoir/RIOT,smlng/RIOT,miri64/RIOT,ant9000/RIOT,lazytech-org/RIOT,OTAkeys/RIOT,josephnoir/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,lazytech-org/RIOT,yogo1212/RIOT,ant9000/RIOT,miri64/RIOT,mfrey/RIOT,authmillenon/RIOT,BytesGalore/RIOT,A-Paul/RIOT,authmillenon/RIOT,kbumsik/RIOT,josephnoir/RIOT,biboc/RIOT,lazytech-org/RIOT,yogo1212/RIOT,BytesGalore/RIOT,RIOT-OS/RIOT,lazytech-org/RIOT,toonst/RIOT,toonst/RIOT,smlng/RIOT,kbumsik/RIOT,gebart/RIOT,biboc/RIOT,yogo1212/RIOT,OTAkeys/RIOT,aeneby/RIOT,toonst/RIOT,smlng/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,cladmi/RIOT,aeneby/RIOT,avmelnikoff/RIOT,OTAkeys/RIOT,cladmi/RIOT,A-Paul/RIOT,mfrey/RIOT,ant9000/RIOT,authmillenon/RIOT,kbumsik/RIOT,BytesGalore/RIOT,biboc/RIOT,kaspar030/RIOT,cladmi/RIOT,kYc0o/RIOT,rfuentess/RIOT,mtausig/RIOT,basilfx/RIOT,kYc0o/RIOT,authmillenon/RIOT,mtausig/RIOT,jasonatran/RIOT,ant9000/RIOT,avmelnikoff/RIOT,OTAkeys/RIOT,rfuentess/RIOT,gebart/RIOT,cladmi/RIOT,rfuentess/RIOT,basilfx/RIOT,biboc/RIOT,smlng/RIOT,mtausig/RIOT,rfuentess/RIOT,aeneby/RIOT,kaspar030/RIOT,A-Paul/RIOT,BytesGalore/RIOT,yogo1212/RIOT,x3ro/RIOT,kaspar030/RIOT,x3ro/RIOT,smlng/RIOT,basilfx/RIOT,miri64/RIOT,kaspar030/RIOT,jasonatran/RIOT,avmelnikoff/RIOT,OlegHahm/RIOT,lazytech-org/RIOT,x3ro/RIOT,authmillenon/RIOT,aeneby/RIOT,josephnoir/RIOT,aeneby/RIOT,mtausig/RIOT,cladmi/RIOT,kbumsik/RIOT,basilfx/RIOT,biboc/RIOT,jasonatran/RIOT,OlegHahm/RIOT,A-Paul/RIOT,mfrey/RIOT,miri64/RIOT,kbumsik/RIOT,kYc0o/RIOT,gebart/RIOT,toonst/RIOT,authmillenon/RIOT,kYc0o/RIOT,gebart/RIOT,OlegHahm/RIOT,A-Paul/RIOT,mfrey/RIOT,x3ro/RIOT