tg3: Discover phy address once

The phy address will not change after it has been identified.  Move the
discovery code to a location that only gets executed once.

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