[libata] sata_mv: clean up DMA boundary issues, turn on 64-bit DMA

The chips covered by sata_mv have a 32-bit DMA boundary they must not
cross, not a 64K boundary.  We are merely limited to a 64K maximum
segment size.  Therefore, the DMA scatter/gather table fill code can be
greatly simplified, and we need not cut in half the S/G table size as
reported to the SCSI layer.

Also, the driver forget to turn on 64-bit DMA at the PCI layer.  All
other data structures (both hardware and software) have been prepped for
64-bit PCI DMA.  It was simply never turned on.  <fingers crossed> let's
see if it still works...

Signed-off-by: Jeff Garzik <jeff@garzik.org>