fix return type of netif_add_ip6_address mismatch

the netif_add_ip6_address function was declared err_t in
src/include/lwip/netif.h, but defined as s8_t (the default value of
err_t) in its implementation in src/core/netif.c.

this causes "conflicting types for 'netif_add_ip6_address'" errors if
err_t is defined differently in cc.h (as for example recommended in
[1]).

as it only returns error constants, it is changed to use err_t
throughout.

[1] http://lwip.wikia.com/wiki/Porting_For_Bare_Metal

src/core/netif.c

@@ -964,7 +964,7 @@ netif_create_ip6_linklocal_address(struct netif *netif, u8_t from_mac_48bit)
  * @param ip6addr address to add
  * @param chosen_idx if != NULL, the chosen IPv6 address index will be stored here
  */
-s8_t
+err_t
 netif_add_ip6_address(struct netif *netif, ip6_addr_t *ip6addr, s8_t *chosen_idx)
 {
   s8_t i;