mxc_nand: fix a problem writing more than 32MB

When writing 0x4000 to the unlockend_blkaddr register, large writes to
a 2k page NAND sometimes fail. The current kernel driver writes 0xFFFF
to this register for V2 of the nand controller.

However on an i.MX31 this also fixes writes larger than 32MB.
The datasheet is very unspecific, but (0x4000=16384)*2000
roughly fits the limits we're encountering with NAND writes.
This problem might be NAND chip specific.

Signed-off-by: Helmut Raiger <helmut.raiger@hale.at>
Signed-off-by: Scott Wood <scottwood@freescale.com>