staging: comedi: ni_daq_700: add AI range and input mode switching

Add support for switching the input range and the single-ended/
differential input mode for the AI subdevice.  We needed to clear the
FIFO of data before the conversion to handle card mode switching
glitches.

[ Minor whitespace fixes and driver comment reformatting.  - Ian ]

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