staging: comedi: addi_apci_3xxx: digital output 'maxdata' is always '1'

Digital output subdevices always have a 'maxdata' of '1'. Remove the
'i_DoMaxData' boardinfo and just open code the value.

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>