[libata] ahci: AMD SB700/SB800 SATA support 64bit DMA

SB700 SATA controller can support 64 bit DMA, the previous commit
badc2341

 was added with
careless reference to SB600, which should be modified by this patch.

Signed-off-by: Shane Huang <shane.huang@amd.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>