V4L/DVB (9231): cx23885: Define num_frontends as a function of the port

This allows some cardcoded functions to be more flexible, and paves
the way for any future cards that may have MFE support. Better to add it
now when the MFE patches is fresh in peoples mind, rather than 12 months
from now when new cards appear.

Signed-off-by: Steven Toth <stoth@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>