gnrc_sixlowpan: stop type-finding loop when type is found

Currently the loop just continues to run after a viable type is found.
In #10851 this lead to a crash of the tests, when the dependency of
`gnrc_sixlowpan` to `gnrc_ipv6` was removed.

sys/net/gnrc/network_layer/sixlowpan/gnrc_sixlowpan.c

@@ -73,6 +73,7 @@ void gnrc_sixlowpan_dispatch_recv(gnrc_pktsnip_t *pkt, void *context,
          ptr = ptr->next) {
         if ((ptr->next) && (ptr->next->type == GNRC_NETTYPE_NETIF)) {
             type = ptr->type;
+            break;
         }
     }
 #else   /* MODULE_GNRC_IPV6 */