tg3: Workaround tagged status update bug

On rare occasions, writing the tag to the interrupt mailbox does not
reenable interrupts.  This patch fixes the problem by reissuing the
mailbox update.

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