staging: comedi: pcmad: properly handle single/diff input configuration

The comedi_config utility is used to attach to this board. One of the
configuration options passed by the user sets the analog input reference
for single-ended or differential inputs.

Use the configuration option to correctly set the comedi_subdevice flags
as well as the number of channels.

Remove the 'differential' variable from the private data. Its not used
by the driver.

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