client: Invalidate scan filter on "scan" command

The "scan bredr" and "scan le" commands are broken right now because the
transport filter gets set, but not actually invalidated.

Invalidate the filter with `filter.set = false` so that it actually gets
updated when set_discovery_filter() is called afterwards.

client/main.c

@@ -1325,9 +1325,14 @@ static void cmd_scan(int argc, char *argv[])
 		return bt_shell_noninteractive_quit(EXIT_FAILURE);
 
 	if (enable == TRUE) {
-		if (strcmp(mode, "")) {
+		if (!g_strcmp0(mode, "")) {
+			g_free(filter.transport);
+			filter.transport = NULL;
+			filter.set = false;
+		} else {
 			g_free(filter.transport);
 			filter.transport = g_strdup(mode);
+			filter.set = false;
 		}
 
 		set_discovery_filter(false);