tty: make everyone's chars_in_buffer return >= 0

The tty line disciplines don't expect tty_operations::chars_in_buffer to
return negative values. Fix the two drivers which violate this.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210302062214.29627-43-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -3056,7 +3056,7 @@ static int gsmtty_chars_in_buffer(struct tty_struct *tty)
 {
 	struct gsm_dlci *dlci = tty->driver_data;
 	if (dlci->state == DLCI_CLOSED)
-		return -EINVAL;
+		return 0;
 	return kfifo_len(&dlci->fifo);
 }
 

drivers/tty/vcc.c

@@ -896,7 +896,7 @@ static int vcc_chars_in_buffer(struct tty_struct *tty)
 	port = vcc_get_ne(tty->index);
 	if (unlikely(!port)) {
 		pr_err("VCC: chars_in_buffer: Failed to find VCC port\n");
-		return -ENODEV;
+		return 0;
 	}
 
 	num = port->chars_in_buffer;