USB: serial: remove redundant write-urb kill

Since commit 1ce7b934

 ("USB: serial: reuse generic write urb and
bulk-out buffer") the port write_urb is simply a pointer to the first
member of write_urbs so there's no need to kill it twice.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>