USB: fix omninet write vs. close race

omninet kills all URBs in close. However write() returns as soon as
the URB has been submitted. Killing the last URB means a race that
can lose that date written in the last call to write().
As a fix this is moved to shutdown().

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>