staging: comedi: ni_stc.h: tidy up G0_G1_Select register and bits

Rename the CamelCase. Convert the inline helper functions into macros.

Tidy up the driver code to use the new macros. For consistency, make
the ni_set_gpct_dma_channel() helper follow the same style as the
ni_set_ai_dma_channel() and ni_set_ao_dma_channel() helpers.

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>