staging: comedi: ni_stc.h: tidy up G_HW_Save_Register

Rename the CamelCase and define the G0 and G1 registers to add clarity
to the mio_regmap tables.

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>