hpsa: refactor hpsa_find_board_params() to encapsulate legacy test

Encapsulate the conditional predicate which tests for legacy controllers
in a separate function and rework the code comments.

Reviewed-by: Scott Teel <scott.teel@pmcs.com>
Signed-off-by: Webb Scales <webbnh@hp.com>
Signed-off-by: Don Brace <don.brace@pmcs.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>