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

SB700 SATA controller can support 64 bit DMA, the previous commit
badc2341579511a247f5993865aa68379e283c5c 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>