staging: comedi: das16: all supported board have digital inputs

All the boards supported by this driver have digital inputs and all
of then use the same function to handle the (*insn_bits) for the
subdevice.

Remove the 'di' member from the boardinfo and always initialize the
digital input subdevice during the board attach.

Tidy up the subdevice init a bit as well as the (*insn_bits) function.
The (*insn_bits) function does not need to clear data[0].

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>