usb-wwan: implement TIOCGSERIAL and TIOCSSERIAL to avoid blocking close(2)

Some devices (ex ZTE 2726) 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.

Signed-off-by: Dan Williams <dcbw@redhat.com>