tty: Fix USB kref leak

When we close we must clear the extra reference we got when we read
port->tty. Setting the port tty NULL will clear the kref held by the driver
but not the one we obtained ourselves while doing the lookup.

Signed-off-by: Alan Cox <[e-mail]>
Tested-by: Helge Hafting <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/usb/serial/usb-serial.c

···
 			if (tty->driver_data)
 				tty->driver_data = NULL;
 			tty_port_tty_set(&port->port, NULL);
+			tty_kref_put(tty);
 		}
 	}