cdc-acm: close race betrween suspend() and acm_softint

Suspend increments a counter, then kills the URBs,
then kills the scheduled work. The scheduled work, however,
may reschedule the URBs. Fix this by having the work
check the counter.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Cc: stable <stable@vger.kernel.org>
Tested-by: Jonas Karlsson <jonas.karlsson@actia.se>
Link: https://lore.kernel.org/r/20200415151358.32664-1-oneukum@suse.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>