staging: comedi: dt2801: fix access to 2nd dio subdevice

Only 4 subdevices are allocated by this driver. The 2nd dio
subdevice is 'dev->subdevice + 3' not '... + 4'.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>