mtd: denali: Disable sub-page writes in Denali NAND driver

The Denali Controller IP does not support sub-page writes.

Signed-off-by: Graham Moore <grmoore@opensource.altera.com>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>