drivers: net: ethernet: cpsw: implement get phy_id via ioctl

Implement get phy_id via ioctl SIOCGMIIPHY. In switch mode active phy_id
is returned and in dual EMAC mode slave's specific phy_id is returned.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>