USB: mos7840: remove unnecessary null test before kfree

Cc: Johan Hovold <jhovold@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Johan Hovold <johan@kernel.org>