V4L/DVB (4757): Cx88: determine whether or not to use external adc

Some cx88-blackbird boards use an external adc, but not necessarily
for all inputs.  Thus, this needs to be configurable on the card level
for each input.
This patch allows for the usage of the external adc to be determined
by a bit setting in the cx88_input struct for cards based on the cx88
blackbird design.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>