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

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: ef5749ef8b30 ("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>