net: IPv6 address should be const in net_if_get_ll_reserve

As the function does not modify the IPv6 address,
it can be const.

Change-Id: I2af58a4954d8b6f0d71bc7d0d14ddf0f62f8ca7c
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>

include/net/yaip/net_if.h

@@ -315,7 +315,7 @@ static inline enum net_verdict net_if_recv_data(struct net_if *iface,
  * @return Return the link layer header size
  */
 static inline uint16_t net_if_get_ll_reserve(struct net_if *iface,
-					     struct in6_addr *dst_ip6)
+					     const struct in6_addr *dst_ip6)
 {
 	return iface->l2->reserve(iface, (void *)dst_ip6);
 }