staging: comedi: ni_mio_common.c: wrap the stc_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 stc registers.

Wrap these operations with some helper functions to clarify the code
a bit and make it easier to read and maintain.

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>