[media] au0828: Add option to preallocate digital transfer buffers

Added command line parameter preallocate_big_buffers so that the digital
transfer buffers can be allocated when the driver is registered. They
do not have to be allocated every time a feed is started.

Signed-off-by: Tim Mester <tmester@ieee.org>
Acked-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>