flash/nor/at91samd: fix chip erase of a secured device

flash/nor/at91samd: fix chip erase of a secured device

'at91samd chip-erase' command did not work on secured device.

Fix it changing address of DSU.CTRL register
(see Atmel SAM D21 datasheet, 13.9. Intellectual Property Protection).

While on it check error return of DSU.CTRL write.

Change-Id: I83155a634a5458cdc0cc16c99c0e155eb1d8b3d6
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reported-by: Thomas Irmen <tirmen@gmx.net>
Reviewed-on: http://openocd.zylin.com/4043
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>