mtd: mxc_nand: move ecc strengh setup before nand_scan_tail

Since commit 6a918bade9dab40aaef80559bd1169c69e8d69cb, the mxc_nand driver
fails with:

Driver must set ecc.strength when using hardware ECC

This is because nand_scan_tail checks for correct ecc strength
settings, so we must set them up before nand_scan_tail.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Cc: stable@vger.kernel.org [3.4+]
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>