[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: NHans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/cx88/cx88-alsa.c

@@ -878,7 +878,7 @@ static int __devinit cx88_audio_initdev(struct pci_dev *pci,
 {
 	struct snd_card  *card;
 	snd_cx88_card_t  *chip;
-	struct cx88_core *core;
+	struct cx88_core *core = NULL;
 	int              err;
 
 	if (devno >= SNDRV_CARDS)