mxc_nand: fix correct_data function

The v2 controller has a totally different mechanism to check
whether the data we read had ecc errors or not. Implement this.
The mechanism in the v2 controller happens to be identical to
the v3 controller.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>