[media] omap3isp: Fix error handling in probe

The mutex was not destroyed correctly if dma_coerce_mask_and_coherent()
failed for some reason.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>