mtd: rawnand: gpmi: pass buffer and len around

Instead of putting the buffer and len passed in from the mtd core
into the private data struct, just pass it around in the GPMI
drivers functions. This makes the lifetime of the variables more
clear and the code easier to follow.

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>