[media] dib7090p: improve the support of the dib7090 and dib7790

The intend of this patch is to improve the support of the dib7090 and
dib7790. The AGC1 min value is set to 32768 by default. The actual AGC1 min
and the external attenuation are controled depending on the received RF
level.

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>