mtd: nand: imx-bbm: use raw reading when checking for factory BBM
This is necessary to prevent the imx_nand_bbm command to bail out
on ECC errors which leaves the device without BBT.
Also simplify buffer management: Use on-stack buffer instead of malloc.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>