kinetis: timer: #ifdef unused functions
kinetis: timer: #ifdef unused functions

Another issue revealed by compiling several apps with LLVM/clang for
the `teensy31` board.

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