[TG3]: Fix 5704 single-port mode

If the dual-port 5704 is configured as a single-port device with
only one PCI function, it would trigger a BUG() condition in
tg3_find_5704_peer(). This fixes the problem by returning its
own pdev if the peer cannot be found.

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