ALSA: usb-audio: Fix copy&paste error in the validator

The recently introduced USB-audio descriptor validator had a stupid
copy&paste error that may lead to an unexpected overlook of too short
descriptors for processing and extension units.  It's likely the cause
of the report triggered by syzkaller fuzzer.  Let's fix it.

Fixes: 57f87706

 ("ALSA: usb-audio: More validations of descriptor units")
Reported-by:  <syzbot+0620f79a1978b1133fd7@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/s5hsgnkdbsl.wl-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>