flash/nor: at91samd modified to use real erase sector size

Before this change SAMD driver defined "sector" equal to a flash
protection block. Oversize sectors (16kB for the biggest flash size)
made problems for flashing firmware split to two or more parts.

Removed superfluous test of sector protection before erase.

Change-Id: I8e6a6bda6ccd91eda2df67ec48270c69faa1bdd1
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/3546


Tested-by: jenkins
Reviewed-by: Stian Skjelstad <stian@nixia.no>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>