stmmac: set ptp_clock to NULL while unregister

This is to properly put to NULL the ptp_clock while un-register the PTP support.
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -206,6 +206,7 @@ void stmmac_ptp_unregister(struct stmmac_priv *priv)
 {
 	if (priv->ptp_clock) {
 		ptp_clock_unregister(priv->ptp_clock);
+		priv->ptp_clock = NULL;
 		pr_debug("Removed PTP HW clock successfully on %s\n",
 			 priv->dev->name);
 	}