[media] cx88-alsa: fix compiler warning

drivers/media/video/cx88/cx88-alsa.c: In function 'cx88_audio_initdev':
drivers/media/video/cx88/cx88-alsa.c:881:20: warning: 'core' may be used uninitialized in this function

The compiler doesn't understand that snd_cx88_create fills in the core
pointer. So just initialize it to NULL.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>