staging: comedi: ni_stc.h: remove stc read/write callbacks from ni_private

The drivers that include ni_mio_common.c set the stc register read/write
callbacks to private functions that handle the read/write operations in
the correct manner for the hardware.

The ni_atmio and ni_mio_cs drivers use identical code to handle the
operations.

The ni_pcimio driver is a bit different due to the non-windowed setup
of the stc registers on the m series boards. For the other boards
supported by the ni_pcimio driver, the direct access of the first 8
STC registers is also disabled due to a difference on the 611x devices.

These differences can all be handled in the ni_stc_{read,write}[lw]()
helpers. Refactor the helpers and remove the callbacks from the private
data.

Also, move the helper functions the handle the mapping of the windowed
STC register offsets to M series register offset from ni_pcimio.c to
ni_mio_common.c.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>