staging: comedi: das1800: cleanup das1800_probe()

Make this function return a pointer to the boardinfo instead of an index.
Since it's not typical, add a comment to clarify why this legacy driver
does a probe.

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>