mtd: rawnand: pasemi: Don't use static data to track per-device state

Up to now the pasemi nand driver only supported a single device
instance. However the check for that was racy because two parallel calls
of pasemi_nand_probe() could pass the check

	if (pasemi_nand_mtd)
		return -ENODEV;

before any of them assigns a non-NULL value to it.

So rework the driver to make use of per-device driver data.

As an intended side effect the driver can bind more than one device and
also gets rid of the check

	if (!pasemi_nand_mtd)
		return 0;

in the remove callback that could only ever trigger after the above race
happened.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230102124051.1508424-1-u.kleine-koenig@pengutronix.de