[media] usbtv: Add S-Video input support

Alongside already existing Composite input.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Mauro Carvalho Chehab <mchehab@redhat.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-media@vger.kernel.org