net: stmmac: set speed at SPEED_UNKNOWN in case of broken speed

In case of invalid speed given, stmmac_adjust_link() still record it as
current speed.
This patch modify the default case to set speed as SPEED_UNKNOWN if not
10/100/1000.

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>