at91smad: Fixes to 'samd_protect'

Some fixes to 'samd_protect' including:
     - Fix a bug in which the value of 'set' parameter passed into the
     function was ignored so it was impossible to remove flash
     protection once it was set.
     - Check the protection status of the sector via 'is_protected'
     field of the  corresponding 'flash_sector' structure to see if
     any actual HW manipulations needs to be done.
     - Change the way the errors during protection activation are
     handled. Now even in the case of error in the middle of
     protecting a number of sectors the subroutine would still update
     the state of the sector protection in sectors array so as to
     avoid cases where openocd thinks that the sector is not protected
     while it actually is.

Change-Id: I4cc6445a98ec13bdd94c89f0711c17840738a215
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-on: http://openocd.zylin.com/2027


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