ALSA: usb-audio: Avoid autopm calls after disconnection

commit 59866da9e4ae54819e3c4e0a8f426bdb0c2ef993 upstream.

Add a similar protection against the disconnection race and the
invalid use of usb instance after disconnection, as well as we've done
for the USB audio PCM.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=51201

Reviewd-by: Clemens Ladisch <clemens@ladisch.de>
Tested-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

 Author:    Takashi Iwai <tiwai@suse.de>
	modified:   sound/usb/midi.c