Clean up and error out if dvb_net_init fails (for example due to
ENOMEM).  This involves moving the dvb_net_init call to before
frontend_init to make cleaning up a little easier.

From an audit of dvb_net_init callers, now that dvb_net_init lets
callers know about errors.
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>