Refactor mbed OS LWIP integration layer

Author: bulislaw

features/FEATURE_LWIP/lwip-interface/emac_lwip.c

@@ -63,7 +63,7 @@ static void emac_lwip_state_change(void *data, bool up)
  * \param[in] action
  * \return ERR_OK or error code
  */
-err_t igmp_mac_filter(struct netif *netif, const ip4_addr_t *group, u8_t action)
+static err_t igmp_mac_filter(struct netif *netif, const ip4_addr_t *group, u8_t action)
 {
     emac_interface_t *emac = netif->state;
     if (emac->ops->add_multicast_group == NULL) {
@@ -104,7 +104,7 @@ err_t igmp_mac_filter(struct netif *netif, const ip4_addr_t *group, u8_t action)
  * \param[in] action
  * \return ERR_OK or error code
  */
-err_t mld_mac_filter(struct netif *netif, const ip6_addr_t *group, u8_t action)
+static err_t mld_mac_filter(struct netif *netif, const ip6_addr_t *group, u8_t action)
 {
     emac_interface_t *emac = netif->state;
     if (emac->ops->add_multicast_group == NULL) {