V4L/DVB (6031): Fix for bug 7819: fixed hotplugging for dvbnet

The first part of dvb_net_close() is just a cut&paste from
dvb_generic_release(), so maybe it would be better to just call
dvb_generic_release() instead?

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Acked-by: Markus Rechberger <mrechberger@gmail.com>
Acked-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>