drivers/atwinc15x0: join multicast groups

I noticed the wifi module would not receive any router advertisements
while an esp8266 did just fine.

Turns out joining the multicast groups was not enabled.
With this it works as expected.

drivers/atwinc15x0/atwinc15x0_netdev.c

@@ -466,6 +466,18 @@ static int _atwinc15x0_set(netdev_t *netdev, netopt_t opt, const void *val,
         case NETOPT_STATE:
             assert(max_len <= sizeof(netopt_state_t));
             return _set_state(dev, *((const netopt_state_t *)val));
+        case NETOPT_L2_GROUP:
+            if (m2m_wifi_enable_mac_mcast((void *)val, 1)) {
+                return -EINVAL;
+            } else {
+                return max_len;
+            }
+        case NETOPT_L2_GROUP_LEAVE:
+            if (m2m_wifi_enable_mac_mcast((void *)val, 0)) {
+                return -EINVAL;
+            } else {
+                return max_len;
+            }
         default:
             return netdev_eth_set(netdev, opt, val, max_len);
     }