ALSA: usb-audio: Fix missing unlock at error path of maxpacksize check

ALSA: usb-audio: Fix missing unlock at error path of maxpacksize check

The recent backport of the upstream commit 05a1fc5efdd8 ("ALSA:
usb-audio: Fix potential overflow of PCM transfer buffer") on the
older stable kernels like 6.12.y was broken since it doesn't consider
the mutex unlock, where the upstream code manages with guard().
In the older code, we still need an explicit unlock.

This is a fix that corrects the error path, applied only on old stable
trees.

Reported-by: Pavel Machek <pavel@denx.de>
Closes: https://lore.kernel.org/aSWtH0AZH5+aeb+a@duo.ucw.cz
Fixes: 98e9d5e33bda ("ALSA: usb-audio: Fix potential overflow of PCM transfer buffer")
Reviewed-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>