libata: fix for sata_mv >64KB DMA segments

Fix bug in sata_mv for cases where the IOMMU layer has merged SG entries
to larger than 64KB. They need to be split up before being sent to
the driver.

Just for simplicity's sake, split up at 64K boundary instead of 64K size,
since that's what the common code does anyway.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>