scsi: dpt_i2o: use dma_set_mask

The driver currently uses pci_set_dma_mask despite otherwise using the
generic DMA API.  Also move the dma_get_required_mask check before actually
setting the dma mask, so that we don't end up with inconsistent settings in
corner cases.

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