USB: serial: remove bogus disconnect test in cleanup

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

The device is never cleared, and cleanup 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>