ipc: make `bar <bar_id> mode|hidden_state` behave as documented

sway-bar(5) says:

> For compatibility with i3, bar mode <mode> [<bar-id>] syntax is
> supported along with the sway only bar <bar-id> mode <mode> syntax.

while the actual behavior is that `bar_cmd_mode` ignores already
selected `config->current_bar` and applies the change to all the
configured bars.

sway/commands/bar/hidden_state.c

@@ -54,7 +54,7 @@ struct cmd_results *bar_cmd_hidden_state(int argc, char **argv) {
 	}
 
 	const char *state = argv[0];
-	if (config->reading) {
+	if (config->current_bar) {
 		error = bar_set_hidden_state(config->current_bar, state);
 	} else {
 		const char *id = argc == 2 ? argv[1] : NULL;

sway/commands/bar/mode.c

@@ -58,7 +58,7 @@ struct cmd_results *bar_cmd_mode(int argc, char **argv) {
 	}
 
 	const char *mode = argv[0];
-	if (config->reading) {
+	if (config->current_bar) {
 		error = bar_set_mode(config->current_bar, mode);
 	} else {
 		const char *id = argc == 2 ? argv[1] : NULL;