staging: comedi: ni_mio_common: remove devpriv macro

The ni_mio_common.c file is #include'd by the ni_atmio, ni_mio_cs,
and ni_pcimio drivers. Those drivers all have a devpriv macro of
this type:

This macro relies on a local variable having a specific name.
Remove its use in all the files by replacing it with a local
variable.

Some of the functions in ni_mio_common.c don't always use the
devpriv variable due to differences in how the low-level i/o
is handled by the driver. Tag the variable in those functions
with __maybe_unused to avoid compile warnings.

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>