[media] drx-j: set it to serial mode by default

Currently, there's just one device using this frontend: PCTV 80e,
and it works on serial mode.

Change the default here to serial mode. If we add more devices,
then this option should be set via config structure.

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