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

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/20260216141209.1849200-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>