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

The drivers that include ni_mio_common.c set the 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 ioport register access and the ni_pcimio driver uses memory mapped
register access. The memory mapped base address is stored in the 'mite'
pointer in the private data which is only allocated and initialized by
the ni_pcimio driver.

Detect the need for memory mapped register access by checking if the
'mite' pointer is set in the private data and remove the callbacks
from the private data.

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>