V4L/DVB (13293): dvb-usb-friio: return the correct DTV_DELIVERY_SYSTEM

This patch makes the driver return the correct DTV_DELIVERY_SYSTEM.

The driver previously returned SYS_UNDEFINED for DTV_DELIVERY_SYSTEM property,
as it lacked any driver specific S2API support.

Signed-off-by: Akihiro Tsukada <tskd2@yahoo.co.jp>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>