[BNX2]: Enable DMA on 5709.

Add missing code to enable DMA on 5709 A1.  The bit is a no-op on A0
and therefore can be set on all 5709 chips.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Acked-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>