mtd/nand/vf610_nfc: Disable subpage writes

This patch disables subpage writes for vf610_nfc nand
driver. This is required, as without this fix, writing
unaligned u-boot images with DFU results in a hang.
Trying to write unalgined binary images also results
in a hang, without disabling subpage writes.

Patch has been tested on a Colibri VF61 module.

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>