cdc-acm: implement TIOCSSERIAL to avoid blocking close(2)

commit ba2d8ce9db0a61505362bb17b8899df3d3326146 upstream.

Some devices (ex Nokia C7) simply don't respond at all when data is sent
to some of their USB interfaces.  The data gets stuck in the TTYs queue
and sits there until close(2), which them blocks because closing_wait
defaults to 30 seconds (even though the fd is O_NONBLOCK).  This is
rarely desired.  Implement the standard mechanism to adjust closing_wait
and let applications handle it how they want to.

See also 02303f73373aa1da19dbec510ec5a4e2576f9610 for usb_wwan.c.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Acked-by: Oliver Neukum <oneukum@suse.de>
Tested-by: Aleksander Morgado <aleksander@gnu.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>