mtd: mxc-nand: Only enable hardware checksumming for fully detected flashes

At least on i.MX25 (i.e. NFCv2) preset_v2 is called with mtd->writesize
== 0 that is before the connect flash chip is detected. It then
configures for 8 bit ECC mode which needs 26 bytes of OOB per 512 bytes
main section. For flashes with a smaller OOB area issuing a read page
command makes the controller stuck with this config.

Note that this currently doesn't hurt because the first read page
command is issued only after detection is complete and preset is called
once more.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>