[libata] ahci: Restore SB600 SATA controller 64 bit DMA

[libata] ahci: Restore SB600 SATA controller 64 bit DMA

Community reported one SB600 SATA issue(BZ #9412), which led to 64 bit
DMA disablement for all SB600 revisions by driver maintainers with
commits 86c9f576a6259148c254d54db3350ef8bf43e270 and
f2499916f390228689961d62c2aadb15d6f2f4e3.

But the root cause is ASUS M2A-VM system BIOS bug in old revisions
like 0901, while forcing into 32bit DMA happens to work as workaround.
Now it's time to withdraw f2499916f390228689961d62c2aadb15d6f2f4e3
so as to restore the SB600 SATA 64bit DMA capability.
This patch is also adding the workaround for M2A-VM old BIOS revisions,
but users are suggested to upgrade their system BIOS to the latest one
if they meet this issue.

Signed-off-by: Shane Huang <shane.huang@amd.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>