scsi: aha1542: convert to DMA mapping API

aha1542 is one of the last users of the legacy isa_*_to_bus APIs, which
also isn't portable enough.  Convert it to the proper DMA mapping API.

Thanks to Ondrej Zary for testing and finding and fixing a crucial bug.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>