stmmac: disable at run-time the EEE if not supported

This patch is to disable the EEE (so HW and timers)
for example when the phy communicates that the EEE
can be supported anymore.

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