staging: comedi: ni_stc.h: add read/write callbacks to struct ni_private

The {read,write}[bwl] macros used to access the registers in the ni_atmio,
ni_mio_cs, and ni_pcimio drivers and the included ni_mio_common.c file all
rely on a local variable having a specific name. They also require some of
the ni_mio_common code to need a __maybe_unused tag on the devpriv local
variable.

Remove all the macros by converting them into private functions and storing
the callbacks in the private data.

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>