staging: comedi: ni_stc.h: tidy up M_Offset_* values

For aesthetics, define the M_Offset_* values instead of using an enum.

Convert the inline helpers used to get some of the M-Series register
offsets into 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>