V4L/DVB (9333): cx88: Not all boards that requires cx88-mpeg has frontends

The multifrontend changes on cx88 assumed that all boards that use cx88-mpeg
supports DVB. This is not true. There also a few analog-only boards based on
Blackboard design that also uses cx88-mpeg. For those boards, there's no need
to allocate dvb frontends.

This patch fixes videobuf allocation for those devices.

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