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>

drivers/staging/comedi/drivers/ni_daq_700.c

@@ -139,6 +139,8 @@ static int daq700_ai_rinsn(struct comedi_device *dev,
 	/* write channel to multiplexer */
 	/* set mask scan bit high to disable scanning */
 	outb(chan | 0x80, dev->iobase + CMD_R1);
+	/* mux needs 2us to really settle [Fred Brooks]. */
+	udelay(2);
 
 	/* convert n samples */
 	for (n = 0; n < insn->n; n++) {