[media] v4l2-mc: Fix parameter description

.//include/media/v4l2-mc.h:138: warning: No description found for parameter 'vdev'
.//include/media/v4l2-mc.h:152: warning: No description found for parameter 'vdev'

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>