mtd: don't use __exit_p to wrap mxcnd_remove

The function mxcnd_remove is defined using __devexit, so don't use
__exit_p but __devexit_p to wrap it.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>