tg3: Move tg3_set_rx_mode

This patch moves __tg3_set_rx_mode above its first use and moves
tg3_set_rx_mode down closer to where the netdev_ops functions should be.

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