V4L/DVB (4667): Changed cx88_board .dvb and .register to an enum.

Some basic cleanup in preperation for a future patch where
the cx88-mpeg functions have to deal with the port being
used by multiple frontends in (mpeg2 hw encoder and dvb demod).

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