mtd: rawnand: meson: fix bitmask for length in command word

mtd: rawnand: meson: fix bitmask for length in command word

Valid mask is 0x3FFF, without this patch the following problems were
found:

1) [    0.938914] Could not find a valid ONFI parameter page, trying
                  bit-wise majority to recover it
   [    0.947384] ONFI parameter recovery failed, aborting

2) Read with disabled ECC mode was broken.

Fixes: 8fae856c5350 ("mtd: rawnand: meson: add support for Amlogic NAND flash controller")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/3794ffbf-dfea-e96f-1f97-fe235b005e19@sberdevices.ru