staging: comedi: das800: cleanup das800_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. For aesthetics, rename the local variable 'board' to 'index.

Consolidate the dev_dbg() messages when a probed boardinfo is used and
remove the extra dev_dbg() message when the boardinfo is not found.

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>