staging: comedi: das16: tidy up 8255 digital i/o subdevice

The 8255 digital i/o subdevice is only available on some of the boards
supported by this driver. Currently the 'i8255_offset' member of the
boardinfo is used to indicate that it is available.

To clarify this support, add a 'has_8255' bit-field flag to the boardinfo.
We can then use this flag to conditionally allocate the space for the
subdevice only when it is needed.

Also, make sure to check that subdev_8255_init() is successful and propagate
any errors.

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>