mtd: nand_base: Removed unnecessary command masking

NAND command, passed to cmd_ctrl(), is masked with 0xff. This patch
removes this since masking is not necessary and masking is not performed
in other places for same call.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>