bhnd: fix build on gcc architectures

"make tinderbox" fails on sparc64 GENERIC-NODEBUG with:
bhnd_subr.c:188: warning: control reaches end of non-void function

Sponsored by:	The FreeBSD Foundation

sys/dev/bhnd/bhnd_subr.c

@@ -184,6 +184,8 @@ bhnd_port_type_name(bhnd_port_type port_type)
 		return ("bridge");
 	case BHND_PORT_AGENT:
 		return ("agent");
+	default:
+		return "unknown";
 	}
 }