ethernet:ti:cpsw: fix phy identification with multiple slaves on fixed-phy

When using more than one slave with ti cpsw and fixed phy the pd->phy_id
will be always zero, but slave_data->phy_id must be unique. pd->phy_id
means a "phy hardware id" whereas slave_data->phy_id means an "unique id",
so we should use pd->addr which has the same unique meaning.

Fixes: 1f71e8c9 ("drivers: net: cpsw: Add support for fixed-link PHY")
Signed-off-by: Pascal Speck <kernel@iktek.de>
Signed-off-by: David S. Miller <davem@davemloft.net>