tg3: use phylib when robo switch is in use

When a switch is connected as a PHY to the MAC driven by tg3, use
phylib and provide the phy address to tg3 from the sprom.

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>