mips: sgi-ip22: Drop redundant check from .remove()

The remove callback is only called by the driver core if there is a
driver to unbind, so there is no need to check dev->driver to be
non-NULL.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>