USB: usblcd: fix I/O after disconnect

Make sure to stop all I/O on disconnect by adding a disconnected flag
which is used to prevent new I/O from being started and by stopping all
ongoing I/O before returning.

This also fixes a potential use-after-free on driver unbind in case the
driver data is freed before the completion handler has run.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Cc: stable <stable@vger.kernel.org>	# 7bbe990c


Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20190926091228.24634-7-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>