serial: 8250_mid: correct comment regarding Tangier HSU

On Intel Penwell and Tangier the HSU block (3 HSU ports) has a global register
set which is currently not used by the driver. On Tangier it has it's own PCI
device and thus available for enumeration. Since it's not a real HSU port we
just skip it and therefore put a comment in the code why we do so.
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_mid.c

@@ -80,7 +80,11 @@ static int tng_setup(struct mid8250 *mid, struct uart_port *p)
 	struct pci_dev *pdev = to_pci_dev(p->dev);
 	int index = PCI_FUNC(pdev->devfn);
 
-	/* Currently no support for HSU port0 */
+	/*
+	 * Device 0000:00:04.0 is not a real HSU port. It provides a global
+	 * register set for all HSU ports, although it has the same PCI ID.
+	 * Skip it here.
+	 */
 	if (index-- == 0)
 		return -ENODEV;