tty: Allow uart_register/unregister/register

This is legitimate but because we don't clear the drv->state pointer in the
unregister code causes a bogus BUG().

Resolves-bug: https://bugzilla.kernel.org/show_bug.cgi?id=42880
Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -2282,6 +2282,7 @@ void uart_unregister_driver(struct uart_driver *drv)
 	tty_unregister_driver(p);
 	put_tty_driver(p);
 	kfree(drv->state);
+	drv->state = NULL;
 	drv->tty_driver = NULL;
 }