[media] v4l: Add subdev sensor g_skip_frames operation

Some buggy sensors generate corrupt frames when the stream is started.
This new operation return the number of corrupt frames to skip when
starting the stream.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>