[media] stv0900: speed up DVB-S searching

Simptoms:
After zapping DVB-S2 channel searching for DVB-S tooks long time.
Now it takes less than a second.

Signed-off-by: Igor M. Liplianin <liplianin@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>