[media] bttv: fix g_tuner capabilities override

The capability field of v4l2_tuner should be ORed by the various subdevs
and by the main driver. In this case the stereo capability was dropped.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>