tg3: add support a phy at an address different than 01

When phylib was in use tg3 only searched at address 01 on the mdio
bus and did not work with any other address. On the BCM4705 SoCs the
switch is connected as a PHY behind the MAC driven by tg3 and it is at
PHY address 30 in most cases. This is a preparation patch to allow
support for such switches.

phy_addr is set to TG3_PHY_MII_ADDR for all devices, which are using
phylib, so this should not change any behavior.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>