nubus: Simplify check in remove callback

The driver core only calls a remove callback when the device was
successfully bound (aka probed) before. So dev->driver is never NULL and
the respective check can just be dropped.

Acked-by: Finn Thain <fthain@linux-m68k.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210730191035.1455248-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>