[media] dm1105: release dvbnet on frontend attachment failure

The patch "dm1105: handle errors from dvb_net_init" moved the
initialization of dvbnet to before frontend attachment but forgot
to adjust the error handling when frontend attachment fails.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>