mtd: rawnand: gpmi: remove direct_dma_map_ok from driver data struct

Instead of putting direct_dma_map_ok into driver struct pass it around
between functions to make the code more readable.

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>