[media] dib7000p: enhancement

The intend of this patch is to improve the support of the dib7000p. It is
now possible to set the minimum value for the AGC1. Also, the driver takes
into account the frequency offset introduced in the tuned frequency.

Signed-off-by: Olivier Grenie <olivier.grenie@parrot.com>
Signed-off-by: Patrick Boettcher <patrick.boettcher@parrot.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>