tg3: Save stats across chip resets

Tg3 has a place to store stats, but doesn't really use it.  This patch
modifies the driver so that stats are saved across chip resets and gets
cleared across close / open calls.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>