tls13: keys: Fix error code

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/library/ssl_tls13_keys.c b/library/ssl_tls13_keys.c
index b1f992e..d8d28f1 100644
--- a/library/ssl_tls13_keys.c
+++ b/library/ssl_tls13_keys.c
@@ -1307,11 +1307,11 @@
         else if( mbedtls_ssl_tls13_named_group_is_dhe( handshake->offered_group_id ) )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "DHE not supported." ) );
-            return( MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE );
+            return( MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE );
         }
     }
 #else
-    return( MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE );
+    return( MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE );
 #endif /* MBEDTLS_KEY_EXCHANGE_SOME_ECDHE_ENABLED */
 
     /*