mtd: rawnand: Propage CS selection to sub operations

Some controller using the instruction parse infrastructure might need
to know which CS a specific sub-operation is targeting. Let's propagate
this information.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200505101353.1776394-2-boris.brezillon@collabora.com