staging: comedi: quatech_daqp_cs: there are 4 digital outputs

This board has 4 digital outputs not 1. Fix the subdevice init.

The digital output lines are used for the external channel selection
when the expansion mode is enabled. Add a comment about this.

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>