[media] stb0899: fix diseqc messages getting lost

- Some badly written applications in conjuction with certain SEC devices
  do send too many messages causing the DiSEqC bus to get congested. As
  a result, the end devices do get confused and hence do not respond.
  The issue was found by Julian Scheel.

Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>