[media] dvbdev: create links on devices with multiple frontends

Devices like mxl111sf-based WinTV Aero-m have multiple
frontends, all linked on the same demod. Currently, the
dvb_create_graph() function is not smart enough to create
multiple links. Fix it.

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