ALSA: usb-audio: rename substream format field to altset_idx

The snd_usb_substream::format field actually contains the index of the
current alternate setting, so rename it to altset_idx to avoid
confusion.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/card.h b/sound/usb/card.h
index 71f03c1..856d71b 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -54,7 +54,7 @@
 	struct audioformat *cur_audiofmt;	/* current audioformat pointer (for hw_params callback) */
 	unsigned int cur_rate;		/* current rate (for hw_params callback) */
 	unsigned int period_bytes;	/* current period bytes (for hw_params callback) */
-	unsigned int format;     /* USB data format */
+	unsigned int altset_idx;     /* USB data format: index of alternate setting */
 	unsigned int datapipe;   /* the data i/o pipe */
 	unsigned int syncpipe;   /* 1 - async out or adaptive in */
 	unsigned int datainterval;	/* log_2 of data packet interval */