net:stmmac: Remove bus_id from mdio platform data.

This patch removes bus_id from mdio platform data, The reason to remove
bus_id is, stmmac mdio bus_id is always same as stmmac bus-id, so there
is no point in passing this in different variable.
Also stmmac ethernet driver connects to phy with bus_id passed its
platform data.
So, having single bus-id is much simpler.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>