at91samd: Bail early if trying to write to protected sector

Bail early if trying to write to protected sector also mark the sector
we are writing to as dirty.

Change-Id: I892f83461792e1cc2dcccade7aa65717831a6805
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-on: http://openocd.zylin.com/2025


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>