mtd: spi-nor: Check that BP bits are set properly

Previously, the lock and unlock functions returned success even if the
BP bits were not actually updated in the status register due to
hardware write protection. Introduce write_sr_and_check() to write and
read back the status register to ensure the desired BP bits are
actually set as requested.

Signed-off-by: Joe Schultz <jschultz@xes-inc.com>
Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>