staging: comedi: ni_tio: convert NI_660x_Up_Down_Pin_Second_Gate_Select() to a macro

The BUG_ON() in this function can never happen.

For aesthetics, rename this CamelCase inline function and convert it into
a simple macro.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>