corruption bug where if an ATA command is issued before DMA is started,
data will become available to the controller before it knows what to do
with it. This results in either data corruption or a controller crash.
This patch remedies the problem by adopting the workaround employed
by Linux and Darwin: starting the DMA engine prior to sending the ATA
command.
Observer on: Xserve G5
Reviewed by: mav
MFC after: 1 week
351129c7bb