[media] Technisat SkyStar USB HD,(DVB-S/S2) too much URBs for arm devices

Using 8 URBs results in a consecutive buffer allocation of too much
memory for some arm devices.
As we use isochronuous transfers the number of URBs can be reduced
without risking data-loss.

Signed-off-by: Christian Loepke <loepke@edfritsch.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>