V4L/DVB (6854): xc2028: be more specific about when applying offset for 7MHz channels

We have been inserting a mystery 500kHz offset for tuning 7MHz channels,
however some experimentation reveals it is only needed under certain
conditions with specific firmware combinations.  Document these and only
apply the offset when we know it is required.

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