scsi: gdth: use generic DMA API

Switch from the legacy PCI DMA API to the generic DMA API.  Also switch
to dma_map_single from pci_map_page in one case where this makes the code
simpler.

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