V4L/DVB (9225): MFE: Add configurable gate control

This adds a configurable (one per card) gate control option
for multi-frontend. Prior to this point gate control was
assumed to be on the primary frontend, this is a fault
when the gate to the analogue section is on the secondary
which is the default for both the HVR-3000 and HVR-4000
in MFE.

Signed-off-by: Darron Broad <darron@kewl.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>