V4L/DVB (9055): tuner-xc2028: Do a better job selecting firmware type

Firmware selection is very tricky on this device. This patch do a better
selection of the proper firmware type, by using a code to hint if the
firmware to be loaded should be D2620 or D2633.

It also allows overriding the hint at the control structure.

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