mtd: mxc_nand: Fix crash after MTD resync

The driver triggered a BUG() in nand_base.c:3214/nand_scan_tail()
because the ecc.strength was not set in NAND_ECC_HW_SYNDROME ECC
mode.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Benoit Thebaudeau <benoit.thebaudeau@advansee.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>