Flash protection set on a device with MANW=1 was lost after reset.
Since #2903 the driver honored MANW bit and issued Write Page command just
for main flash write. This change adds similar technique to
samd_modify_user_row().

Minor code improvements:
samd_check_error() returns error code corresponding to error type
instead of bool.

samd_check_error() does not clear STATUS register if no error bit is set.

Eliminated double error check in call sequence samd_issue_nvmctrl_command()
folowed by samd_check_error().

Missing error code ERROR_FLASH_PROTECTED added to src/flash/common.h.

Change-Id: Icf59ab8803305d0cb3170c8a5089b8f9828b99f8
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/3550


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>