net: phy: mscc: macsec: reject PN update requests

Updating the PN is not supported.
Return -EINVAL if update_pn is true.

The following command succeeded, but it should fail because the driver
does not update the PN:
ip macsec set macsec0 tx sa 0 pn 232 on

Fixes: 28c5107a ("net: phy: mscc: macsec support")
Signed-off-by: Radu Pirea (NXP OSS) <radu-nicolae.pirea@oss.nxp.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/phy/mscc/mscc_macsec.c

@@ -849,6 +849,9 @@ static int vsc8584_macsec_upd_rxsa(struct macsec_context *ctx)
 	struct macsec_flow *flow;
 	int ret;
 
+	if (ctx->sa.update_pn)
+		return -EINVAL;
+
 	flow = vsc8584_macsec_find_flow(ctx, MACSEC_INGR);
 	if (IS_ERR(flow))
 		return PTR_ERR(flow);
@@ -900,6 +903,9 @@ static int vsc8584_macsec_upd_txsa(struct macsec_context *ctx)
 	struct macsec_flow *flow;
 	int ret;
 
+	if (ctx->sa.update_pn)
+		return -EINVAL;
+
 	flow = vsc8584_macsec_find_flow(ctx, MACSEC_EGR);
 	if (IS_ERR(flow))
 		return PTR_ERR(flow);