sam0/timer: various fixes
sam0/timer: various fixes

This cleans up the sam0 timer driver:

 - remove the check for the unused freq parameter
 - the MCU provides dedicated SET/CLR registers to avoid
   read-modify-write, so don't do read-modify-write on them.
 - workaround a possible hardware bug on SAMD5x:
   loop until the CMD_READSYNC is really set

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