staging: comedi: ni_stc.h: tidy up AI_AO_Select register and bits

Rename the CamelCase and convert the enum into macros.

Tidy up the driver code to use the new macros.

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>