ALSA: usb-audio: Don't avoid stopping the stream at disconnection

In the later patch, we're going to issue the PCM sync_stop calls at
disconnection.  But currently the USB-audio driver can't handle it
because it has a check of shutdown flag for stopping the URBs.  This
is basically superfluous (the stopping URBs are safe at disconnection
state), so let's drop the check.

Fixes: dc5eafe7 ("ALSA: usb-audio: Support PCM sync_stop")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210206203052.15606-4-tiwai@suse.de


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