tg3: Fix Read DMA workaround for 5719 A0.

commit 10ce95d6ef36c65df7dcd3b8fcf86913f8b298bd upstream.

The workaround was mis-applied to all 5719 and 5720 chips.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>