[media] cx88-dvb avoid dangling core->gate_ctrl pointer

dvb_register calls videobuf_dvb_register_bus, but if that returns
a failure the module will be unloaded without clearing the
value of core->gate_ctrl which will cause an oops in macros
called from video_open in cx88-video.c

Signed-off-by: David Fries <David@Fries.net>
Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
Cc: Istvan Varga <istvan_v@mailbox.hu>
Cc: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>