ALSA: usb-audio: Use the right limit for PCM OOB check

The recent fix commit for addressing the OOB access of PCM URB data
buffer caused a regression on Behringer UMC2020HD device, resulting in
choppy sound.  The fix used ep->max_urb_frames for the upper limit
check, and this is no right value to be referred.

Use the actual buffer size (ctx->buffer_size) as the upper limit
instead, which also avoids the regression on the device above.

Fixes: ef5749e ("ALSA: usb-audio: Prevent excessive number of frames")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=220997
Link: https://patch.msgid.link/20260121082025.718748-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: tiwai

sound/usb/pcm.c

@@ -1553,7 +1553,8 @@ static int prepare_playback_urb(struct snd_usb_substream *subs,
 
 		for (i = 0; i < ctx->packets; i++) {
 			counts = snd_usb_endpoint_next_packet_size(ep, ctx, i, avail);
-			if (counts < 0 || frames + counts >= ep->max_urb_frames)
+			if (counts < 0 ||
+			    (frames + counts) * stride > ctx->buffer_size)
 				break;
 			/* set up descriptor */
 			urb->iso_frame_desc[i].offset = frames * stride;