spin_is_locked() always returns zero when spinlock debugging is
disabled on a single CPU machine. Replace WARN_ON() with
assert_spin_locked().

Thanks to Laurent Pinchart for spotting this!
Signed-off-by: Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>