zorro: Drop useless (and hardly used) .driver member in struct zorro_dev

The only actual use is to check in zorro_device_probe() that the device
isn't already bound. The driver core already ensures this however so the
check can go away which allows to drop the then assigned-only member
from struct zorro_dev.

If the value was indeed needed somewhere it can always be calculated by

	to_zorro_driver(z->dev.driver)

.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210730191035.1455248-5-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>