USB: usb_wwan: remove bogus disconnect test in close

Remove bogus (and unnecessary) test for serial->dev being NULL in close.

The device is never cleared, and close is never called after a completed
disconnect anyway.

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