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: 28c5107aa904 ("net: phy: mscc: macsec support")
Signed-off-by: Radu Pirea (NXP OSS) <[e-mail]>
Reviewed-by: Sabrina Dubroca <[e-mail]>
Signed-off-by: Paolo Abeni <[e-mail]>

drivers/net/phy/mscc/mscc_macsec.c

···
 	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);
···
 {
 	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))