staging: comedi: quatech_daqp_cs: there are 4 digital inputs

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

The digital input lines are shared with other functions. 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>