At closing a USB MIDI output substream, there might be still a pending
work, which would eventually access the rawmidi runtime object that is
being released.  For fixing the race, make sure to cancel the pending
work at closing.

Reported-by: syzbot+6912c9592caca7ca0e7d@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/000000000000e7e75005dfd07cf6@google.com
Link: https://lore.kernel.org/r/20220525131203.11299-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>