em28xx: fix uninitialized variable warning

Fix this daily build warning:

In file included from build/media_build/v4l/em28xx-core.c:35:0:
build/media_build/v4l/em28xx-core.c: In function 'em28xx_audio_setup':
build/media_build/v4l/em28xx.h:798:2: warning: 'vid' may be used uninitialized in this function [-Wmaybe-uninitialized]
printk(KERN_INFO "%s: "fmt,\
^
build/media_build/v4l/em28xx-core.c:507:6: note: 'vid' was declared here

  u32 vid;
      ^

As far as I can tell 'vid' can not really be used uninitialized here, but the code is sufficiently complex that apparently gcc can't figure that out.

index b5e52fe..901cf2b 100644
@@ -504,7 +504,7 @@ EXPORT_SYMBOL_GPL(em28xx_audio_analog_set);
 int em28xx_audio_setup(struct em28xx *dev)
 {
 	int vid1, vid2, feat, cfg;
-	u32 vid;
+	u32 vid = 0;
 	u8 i2s_samplerates;
 
 	if (dev->chip_id == CHIP_ID_EM2870 ||