ahci: disable 64bit DMA by default on SB600s

ahci: disable 64bit DMA by default on SB600s

Till now only one board, ASUS M2A-VM, can do 64bit dma with recent
BIOSen.  Enabling 64bit DMA by default already broke three boards.
Enabling 64bit DMA isn't worth these regressions.  Disable 64bit DMA
by default and enable it only on boards which are known to work.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Gabriele Balducci <balducci@units.it>
Reported-by: maierp@informatik.tu-muenchen.de
Cc: Shane Huang <shane.huang@amd.com>
Cc: stable@kernel.org
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>