stmmac: Stop advertising 1000Base capabilties for non GMII iface (V4).

This patch stops advertising 1000Base capablities if GMAC is either
configured for MII or RMII mode and on board there is a GPHY plugged on.
Without this patch if an GBit switch is connected on MII interface,
Ethernet stops working at all.

Discovered as part of
https://bugzilla.stlinux.com/show_bug.cgi?id=14148 triage

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