staging: comedi: ni_mio_common: refactor m-series stc register handling

For M-Series boards (devpriv->is_m_series), the STC registers need to be
remapped. This is currently handled with some big switch statements to
convert the STC register offset to the M-Series offset. Some of the
registers also need special handling due to differences in the register
size on the M-Series boards.

Create some lookup tables to handle the remapping of the read and write
registers. These are easier to maintain and can contain the register size
so that a common function can be used instead of the separate 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>