staging: comedi: ni_mio_common.c: wrap the read/write operations

The ni_atmio, ni_pcimio, and ni_mio_cs drivers all include this source
file to handle the common functionality of the drivers. Each of those
drivers set some function pointers in the private data to handle the
read/write operations to the normal registers.

Like done for the stc register operations, wrap these operations with
some helper functions to clarify the code a bit and make it easier to
read and maintain.
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>