bgpd: Fix unhandled enum warning for AFI_ETHER

* bgp_open.c: (bgp_afi_safi_valid_indices) We have AFI_ETHER defined, but
  don't handle it at this point.
diff --git a/bgpd/bgp_open.c b/bgpd/bgp_open.c
index 7b8b657..2800423 100644
--- a/bgpd/bgp_open.c
+++ b/bgpd/bgp_open.c
@@ -143,6 +143,8 @@
 	case SAFI_ENCAP:
 	  return 1;
 	}
+    case AFI_ETHER:
+    default:
       break;
     }