USB: serial: wake up MSR-wait queue on disconnect

Make sure processes waiting for modem-status changes are woken up at
disconnect.

This is needed for custom subdriver TIOCMIWAIT-implementations which do
not yet handle hangup.

Even though processes on the tty-port wait queue are woken up at hangup
the wake-up call in usb-serial disconnect is still needed if a woken-up
process may go back to sleep (e.g. due to an incomplete
TIOCMIWAIT-implementation). If a disconnect occurs after a hangup, any
process waiting for changes will not be woken up a second time by the
tty-layer as the port will then have been disassociated from the tty.
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/usb-serial.c

@@ -1093,6 +1093,7 @@ static void usb_serial_disconnect(struct usb_interface *interface)
 				tty_kref_put(tty);
 			}
 			usb_serial_port_poison_urbs(port);
+			wake_up_interruptible(&port->port.delta_msr_wait);
 			cancel_work_sync(&port->work);
 			if (device_is_registered(&port->dev))
 				device_del(&port->dev);