V4L/DVB (11331): em28xx: convert to v4l2_subdev

Converted em28xx driver to v4l2_subdev.
Thanks to Hans Verkuil <hverkuil@xs4all.nl> for helping this conversion.

Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>