staging: comedi: addi_apci_3xxx: refactor 'i_NbrD[io]Channel' boardinfo

The boards that have digital input and output channels always have
4 inputs and 4 outputs.

Remove the 'i_NbrD[io]Channel' data from the boardinfo and add two
bit fields to indicate that the board 'has_dig_in' and 'has_dig_out'.
Use those to conditionally initialize the subdevices and open code
the s->n_chan value of 4 for both subdevices.

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>