[media] em28xx-audio: fix return code on device disconnect

Alsa has an special non-negative return code to indicate device removal
at snd_em28xx_capture_pointer(). Use it, instead of an error code.
Reviewed-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Tested-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/usb/em28xx/em28xx-audio.c

@@ -434,7 +434,7 @@ static snd_pcm_uframes_t snd_em28xx_capture_pointer(struct snd_pcm_substream
 
 	dev = snd_pcm_substream_chip(substream);
 	if (dev->disconnected)
-		return -ENODEV;
+		return SNDRV_PCM_POS_XRUN;
 
 	spin_lock_irqsave(&dev->adev.slock, flags);
 	hwptr_done = dev->adev.hwptr_done_capture;