[media] videobuf-dvb: avoid spurious ENOMEM when CONFIG_DVB_NET=n

videobuf_dvb_register_bus relies on dvb_net_init to set dvbnet->dvbdev
on success, but ever since commit fcc8e7d8 ("dvb_net: Simplify the
code if DVB NET is not defined"), ->dvbdev is left unset when
networking support is disabled.  Therefore in such configurations
videobuf_dvb_register_bus always returns failure, tripping
little-tested error handling paths and preventing the device from
being initialized and used.

Now that dvb_net_init returns a nonzero value on error, we can use
that as a more reliable error indication.  Do so.

Now your card be used with CONFIG_DVB_NET=n, and the kernel will pass
on a more useful error code describing what happened when
CONFIG_DVB_NET=y but dvb_net_init fails due to resource exhaustion.

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