[media] stv090x: Fix losing lock in dual DVB-S2 mode

Do not clear registers ACLC/BCLC in DVB-S2 mode for Cut <= 20.
Otherwise, the demod could lose the lock periodically.
Verified with cineS2 and Duoflex.

Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>