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>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -739,6 +739,7 @@ static void stmmac_adjust_link(struct net_device *dev)
 			default:
 				netif_warn(priv, link, priv->dev,
 					   "broken speed: %d\n", phydev->speed);
+				phydev->speed = SPEED_UNKNOWN;
 				break;
 			}