V4L/DVB (10562): bttv: rework the way digital inputs are indicated

The code was using a muxsel value of -1U to indicate a digital input.  A
couple places in were checking of muxsel < 0 to detect this, which doesn't
work of course because muxsel is unsigned and can't be negative.

Only a couple cards had digital inputs and it was always the last one, so
for the card database create a one bit field that indicates the last input
is digital.  On init, this is used to set a new field in the bttv struct to
the digital input's number or UNSET for none.  This makes it easier to
check if the current input is digital.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>