mtd: gpmi: do not use the mtd->writesize

The nfc_geo->payload_size is equal to the mtd->writesize now,
use the nfc_geo->payload_size to replace the mtd->writesize.

This patch makes preparation for the gpmi's subpage read support.
In the subpage support, the nfc_geo->payload_size maybe smaller then
the mtd->writesize.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>