mtd: rawnand: tmio: Do not abuse nand_controller->wq

nand_controller->wq has never been meant to be used by NAND controller
drivers. This waitqueue is used by the framework to serialize accesses
to a NAND controller, and messing up with its state is a really bad
idea.

Declare a completion object in tmio_nand and use it to wait for RB
transitions.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>