staging: comedi: das16: remove 'ai_unipolar' and 'ai_singleended'

These members in the private data are only used during the board attach.

Move the reading of the status register to the board attach and just
check the value where needed when initializing the analog input 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>