V4L/DVB (6641): xc2028: correct tuner offset for 7MHz DTV

7MHz bandwidth DVB-T needs an adjusted offset at the PLL to ensure the IF
output is correctly centered.

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