staging: comedi: me_daq: allow differential analog inputs

The ai (*insn_read) allows differential analog inputs but the subdevice
initialization is missing the SDF_DIFF subdev_flag to allow the user to
select this mode. Fix the subdevice init.

According to the manual, differential operation only works with the
first 8 channels and bipolar ranges. Add a check to the (*insn_read) to
ensure this.

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>