USB: serial: clean up chars_in_buffer

No need to grab disconnect mutex in chars_in_buffer now that no
sub-driver is or should be querying hardware buffers anymore. (They
should use wait_until_sent.)
Signed-off-by: NJohan Hovold <jhovold@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/usb-serial.c

@@ -359,20 +359,13 @@ static int serial_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct usb_serial *serial = port->serial;
-	int count = 0;
 
 	dev_dbg(tty->dev, "%s\n", __func__);
 
-	mutex_lock(&serial->disc_mutex);
-	/* if the device was unplugged then any remaining characters
-	   fell out of the connector ;) */
 	if (serial->disconnected)
-		count = 0;
-	else
-		count = serial->type->chars_in_buffer(tty);
-	mutex_unlock(&serial->disc_mutex);
+		return 0;
 
-	return count;
+	return serial->type->chars_in_buffer(tty);
 }
 
 static void serial_wait_until_sent(struct tty_struct *tty, int timeout)