drivers/mtd_flashpage: fix erasure of multiple sectors
drivers/mtd_flashpage: fix erasure of multiple sectors

miri64/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,authmillenon/RIOT,kaspar030/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,miri64/RIOT,kYc0o/RIOT,miri64/RIOT,kYc0o/RIOT,authmillenon/RIOT,miri64/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,jasonatran/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,kaspar030/RIOT,jasonatran/RIOT,authmillenon/RIOT,jasonatran/RIOT,kYc0o/RIOT,kaspar030/RIOT,authmillenon/RIOT,OlegHahm/RIOT,kYc0o/RIOT,ant9000/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,authmillenon/RIOT,OTAkeys/RIOT,authmillenon/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT