staging: comedi: ni_daq_700: add mux settling delay

I got a patch from the original author, Fred Brooks, to add a small
settling delay after setting the AI channel multiplexor.  The lack of
delay resulted in unstable or scrambled data on faster processors.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reported-by: Fred Brooks <nsaspook@nsaspook.com>
Cc: <stable@vger.kernel.org> # 3.7.x - 3.15.x
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>