tg3: Fix int generation hw bug for 5719 / 5720

On the 5719 and 5720, there is a bug where the hardware will
misinterpret a status tag update and leave interrupts permanently
disabled.  This patch enables a hardware fix that works around the
issue.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>