drivers/mtd: fix NULL pointer dereference in partition lock/unlock
Some mtd device does not support lock and unlock functions. Adding this check
avoids crashing when mtd_part_lock/unlock are called for such devices.
Signed-off-by: Zahari Doychev <zahari.doychev@linux.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>