staging: comedi: pcl812: tidy up digital subdevice boardinfo

For the board types that have digital inputs and outputs there are always
16 input channels and 16 output channels.

Remove the 'n_dichan' and 'n_dochan' members in the boardinfo and replace
them with a bit-field flag 'has_dio'. Refactor pcl812_attach() to use the
new boardinfo.

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>