[libata] sata_svw: Disable ATAPI DMA on current boards (errata workaround)

Current Broadcom/Serverworks SATA boards (including Apple K2 SATA)
have problems with ATAPI DMA, so it is disabled.  ATAPI PIO, ATA PIO,
and ATA DMA continue to work just fine.

Acked-by: Anantha Subramanyam <ananth@broadcom.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>