staging: comedi: ni_660x: remove inline mite_ring()

This fuction just returns a pointer from the private data. The name
might provide some confusion since it appears to be an exported
function from the mite driver.

Just remove it and get the pointer directly where needed.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>