ALSA: usb-audio: Avoid dropping MIDI events at closing multiple ports

ALSA: usb-audio: Avoid dropping MIDI events at closing multiple ports

We fixed the UAF issue in USB MIDI code by canceling the pending work
at closing each MIDI output device in the commit below.  However, this
assumed that it's the only one that is tied with the endpoint, and it
resulted in unexpected data truncations when multiple devices are
assigned to a single endpoint and opened simultaneously.

For addressing the unexpected MIDI message drops, simply replace
cancel_work_sync() with flush_work().  The drain callback should have
been already invoked before the close callback, hence the port->active
flag must be already cleared.  So this just assures that the pending
work is finished before freeing the resources.

Fixes: 0125de38122f ("ALSA: usb-audio: Cancel pending work at closing a MIDI substream")
Reported-and-tested-by: John Keeping <jkeeping@inmusicbrands.com>
Closes: https://lore.kernel.org/20250217111647.3368132-1-jkeeping@inmusicbrands.com
Link: https://patch.msgid.link/20250218114024.23125-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>