net: stmmac: use SPEED_UNKNOWN/DUPLEX_UNKNOWN

It is better to use DUPLEX_UNKNOWN instead of just "-1".
Using 0 for an invalid speed is bad since 0 is a valid value for speed.
So this patch replace 0 by SPEED_UNKNOWN.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Reviewed-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>