staging: comedi: adv_pci1710: simplify digital input and output boardinfo

The boards that have digital inputs and outputs always have 16 input channels and
16 output channels.

Replace the 'n_dichan' and 'n_dochan' members of the boardinfo with the fit-field
flag 'has_di_do' and refactor the board attach accordingly.

Remove the unnecessary initialization of the subdevice 'len_chanlist'. That
member is only used by subdevices that support async commands.

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>