staging: comedi: ni_tio: tidy up Gi_Command_Reg_Bits

Convert this enum into defines and rename all the CamelCase symbols.

For aesthetics, move the new defines so they are associated with the
register define.

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>