staging: comedi: das1800: remove 'common' boardinfo member

The "hc" type boards do not support common mode analog inputs
all the other board types do.

Remove the unnecessary member from the boardinfo and use the
'id' member to determine if the SDF_COMMON flag should be set
for the subdevice.

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>