[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 c7a42156d99bcea7f8173ba7a6034bbaa2ecb77c and
4cde32fc4b32e96a99063af3183acdfd54c563f0.

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 4cde32fc4b32e96a99063af3183acdfd54c563f0
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>