staging: comedi: ni_usb6501: rename ni6501_send_command()

Rename ni6501_send_command to ni6501_port_command

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>