tg3: Fix stats while interface is down

If the tg3 interface is down, the driver will return ethtool stats
uninitialized.  This patch zeroes out the destination stat buffer in
such a case.

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