V4L/DVB (6855): xc2028: select DTV78 firmware if tuning 7MHz VHF / 8MHz UHF

It seems that the DTV78 firmware is intended for use in locations where
VHF channels have 7MHz bandwidth and UHF channels have 8MHz bandwidth.
If we switch to DTV78 firmware when we detect this condition, we can
avoid firmware reloads when switching between VHF and UHF transponders.

Place the state for this in the control structure so that card drivers
can hint to us to use DTV78 firmware from the first tuning attempt.

Signed-off-by: Chris Pascoe <c.pascoe@itee.uq.edu.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>