octeontx2-pf: mcs: update PN only when update_pn is true

When updating SA, update the PN only when the update_pn flag is true.
Otherwise, the PN will be reset to its previous value using the
following command and this should not happen:
$ ip macsec set macsec0 tx sa 0 on

Fixes: c54ffc73 ("octeontx2-pf: mcs: Introduce MACSEC hardware offloading")
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/ethernet/marvell/octeontx2/nic/cn10k_macsec.c

@@ -1357,10 +1357,12 @@ static int cn10k_mdo_upd_txsa(struct macsec_context *ctx)
 
 	if (netif_running(secy->netdev)) {
 		/* Keys cannot be changed after creation */
-		err = cn10k_write_tx_sa_pn(pfvf, txsc, sa_num,
-					   sw_tx_sa->next_pn);
-		if (err)
-			return err;
+		if (ctx->sa.update_pn) {
+			err = cn10k_write_tx_sa_pn(pfvf, txsc, sa_num,
+						   sw_tx_sa->next_pn);
+			if (err)
+				return err;
+		}
 
 		err = cn10k_mcs_link_tx_sa2sc(pfvf, secy, txsc,
 					      sa_num, sw_tx_sa->active);
@@ -1529,6 +1531,9 @@ static int cn10k_mdo_upd_rxsa(struct macsec_context *ctx)
 		if (err)
 			return err;
 
+		if (!ctx->sa.update_pn)
+			return 0;
+
 		err = cn10k_mcs_write_rx_sa_pn(pfvf, rxsc, sa_num,
 					       rx_sa->next_pn);
 		if (err)