V4L/DVB (10564): bttv: fix external mux for PHYTEC VD-009

Old versions of the bttv driver would use the high nibble of an input's
muxsel value to program the GPIO lines enabled via gpiomask2.  Apparently
this was supposed to be for switching external audio muxes.  Anyway, the
code that did this was removed sometime in the pre-git 2.6 series.

These phytec boards used this feature to control an external video mux and
I guess no one noticed when they removed the code.

So add a muxsel_hook for these boards that does the necessary gpio setting.

BTW, I doubt the needs_tvaudio setting for these cards is correct.

Cc: Dirk Heer <d.heer@phytec.de>
Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>