mxc_nand: Fix OOB handling

The OOB handling in the mxc_nand driver is broken for v1 type
controllers (i.MX27/31) with 512 byte page size. This perhaps
did not show up because ubi does not use OOB.
Update the driver to always read/write a whole page even if
only OOB is requested. With this patch the driver passes the
mtd_oobtest on i.MX27 with 512 byte page size. Also tested
with 2048 byte page size and on i.MX35 (v2 type controller)

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reported-by: Wolfram Sang <w.sang@pengutronix.de>
Tested-by: Baruch Siach <baruch@tkos.co.il>