ad0 READ_DMA TIMEOUT errors on install of 7.0-RELEASE

[Stephen Hurd]

> last night. It's a MiniITX board, model EN1200. My system can't remain
> up for more than 10minutes before something locks it up and frequently
> the screen will output error messages relating to DMA.

As a workaround, adding the line:
hw.ata.ata_dma="0"

To /boot/loader.conf will disable DMA and prevent the hangs that are caused by the DMA timeouts.