MFC r208870:
Some revisions of the Serverworks K2 SATA controller have a data
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.

Reviewed by: mav
Approved by: re (kib)