cpu/atmega_common: Fixed atmega_exit_isr
cpu/atmega_common: Fixed atmega_exit_isr

A context switch at the end of the ISR should only occur, if requested.
This fixes this.

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