[media] media: i2c: mt9p031: make sure we destroy the mutex

Make sure to call mutex_destroy() in case of probe failure or module
unload.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>