V4L/DVB (6619): Use MTS firmware for the HVR-900

The HVR-900 requires the MTS version of the xc3028 firmware in order
to get any sound. The below patch selects this firmware variant on
HVR-900 cards, as well as splitting the HVR-950 into its own entry
(since I don't know if it uses the MTS variant and it will have to be
split off eventually anyway).

Signed-off-by: Aidan Thornton <makosoft@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>