mtd: rawnand: gpmi: Remove unnecessary variables

this->page_buffer_virt and this->payload_virt are always set to the same
value, so drop the former and just use the latter. Same for
this->page_buffer_virt and this->payload_virt.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>