ALSA: usb-audio: Correct document for snd_usb_endpoint_free_all()

The kerndoc comment for the new function snd_usb_endpoint_free_all()
had a typo wrt the argument name.  Fix it.

Fixes: 00272c61

 ("ALSA: usb-audio: Avoid unnecessary interface re-setup")
Reported-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210205082837.6327-1-tiwai@suse.de


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