mtd: do not use mtd->lock_user_prot_reg directly

Instead, check the -EOPNOTSUPP return code of 'mtd_lock_user_prot_reg()'.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>