ALSA: usb-audio: Update the number of packets properly at receiving

At receiving the packets from the implicit feedback source, we didn't
update ctx->packets field but only the ctx->packet_size[] data.
In exceptional cases, this might lead to unexpectedly superfluous data
transfer (although this won't happen usually due to the nature of USB
isochronous transfer).  Fix it to update the field properly.

Link: https://patch.msgid.link/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>

sound/usb/endpoint.c

@@ -481,6 +481,7 @@ int snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
 
 		/* copy over the length information */
 		if (implicit_fb) {
+			ctx->packets = packet->packets;
 			for (i = 0; i < packet->packets; i++)
 				ctx->packet_size[i] = packet->packet_size[i];
 		}