ALSA: usb-audio: Avoid unnecessary interface change at EP close

We toggle USB interface at PCM prepare and reset at close.  When the
PCM isn't prepared, resetting again makes little sense.
Check the current altset and avoid unnecessary interface reset at EP
close.

Link: https://lore.kernel.org/r/20221009104212.18877-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>