staging: comedi: ni_usb6501: cleanup response_size

Cleanup response_size in ni6501_send_command (READ_PORT command).
No logical/functional change is introduced by this patch.

Signed-off-by: Luca Ellero <luca.ellero@brickedbrain.com>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>