cpu/lm4f120: adapt timer to return 0 on success
cpu/lm4f120: adapt timer to return 0 on success

Adapt periph/timer implementation of lm4f120 based MCUs
to return 0 on success for all functions.

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