[media] siano: honour per-card default mode

Instead of using a global default_mode, passed via modprobe
parameter, use the one defined inside the cards struct.
That will prevent the need of manually specify it for each
board, except, of course, if the user wants to do something
different, on boards that accept multiple types.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>