staging: comedi: ni_660x: remove inline private() function

The inline private() function simply returns the dev->private pointer
to the private data.

Remove the inline function and just use a local variable where the
private data is used.
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>