staging: comedi: addi_apci_3120: remove test for DI subdevice

The boards supported by this driver all have digital inputs.
Remove the test for it.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>