[media] cx231xx: Use a generic check for TUNER_XC5000

The check for xc5000 assumes that the tuner will always
be using the same bus and will have the same address.
As those are configurable via dev->board, it should use,
instead, the values defined there.

Also, a similar type of test will be needed by other
tuners (for example, for TUNER_XC2028)

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>