V4L/DVB: STV0900 Cut 3.0 AGC2 fix for NetUP Dual DVB-S2-CI card

if the AGC2 > 1400 then no signal on this frequency for STV0900 Cut 3.0. For
Cut 2.0 this value is 700.

Signed-off-by: Abylay Ospan <aospan@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>