staging: comedi: addi_apci_3120: remove boardinfo 'i_NbrDoChannel'

All boardinfo entries have this member set as '4'. Remove the member and
initialize the Digial Output 'n_chan' with 4 directly.

Also, remove the initialization of 'len_chanlist'. This member is only
used with 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>