staging: comedi: ni_usb6501: remove useless check

Remove useless test in ni6501_send_command. The check is useless since this
function is called only in this driver.

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>