[TG3]: Fix peer device handling

Locate the pdev_peer for dual port 5714 NIC devices in addition to
5704 devices. The name is also changed to tg3_find_peer() from
tg3_find_5704_peer(). It is also necessary to call netdev_priv() to
get to the peer's private tg3 structure.

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