ALSA: usb-audio: Replace slave/master terms

Follow the inclusive terminology, just replace sync_master/sync_slave
with sync_source/sync_sink.  It's also a bit clearer from its meaning,
too.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-34-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/card.h b/sound/usb/card.h
index 1dd7a51..3e16b92 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -71,8 +71,8 @@ struct snd_usb_endpoint {
 				 struct urb *urb);
 
 	struct snd_usb_substream *data_subs;
-	struct snd_usb_endpoint *sync_master;
-	struct snd_usb_endpoint *sync_slave;
+	struct snd_usb_endpoint *sync_source;
+	struct snd_usb_endpoint *sync_sink;
 
 	struct snd_urb_ctx urb[MAX_URBS];