mtd: rawnand: gpmi: remove unnecessary variables

Use this->auxiliary_virt and this->auxiliary_phys directly rather
than creating extra local variables for them.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>