ALSA: usb-audio: Pre-calculate buffer byte size

There are a bunch of lines calculating the buffer size in bytes at
each time.  Keep the value in subs->buffer_bytes and use it
consistently for the code simplicity.

Link: https://lore.kernel.org/r/20210601162457.4877-3-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/card.h b/sound/usb/card.h
index a741e7d..b346653d 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -158,6 +158,7 @@ struct snd_usb_substream {
 
 	unsigned int running: 1;	/* running status */
 
+	unsigned int buffer_bytes;	/* buffer size in bytes */
 	unsigned int hwptr_done;	/* processed byte position in the buffer */
 	unsigned int transfer_done;		/* processed frames since last period update */
 	unsigned int frame_limit;	/* limits number of packets in URB */