serial: amba-pl011: fix regression, causing an Oops on rmmod

A recent commit ef2889f7 "serial: pl011:
Move uart_register_driver call to device probe" introduced a regression,
causing the pl011 driver to Oops if more than 1 port have been probed. Fix
the Oops by only calling uart_unregister_driver() once after the last port
has been removed.
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/amba-pl011.c

@@ -2176,6 +2176,7 @@ static int pl011_probe(struct amba_device *dev, const struct amba_id *id)
 static int pl011_remove(struct amba_device *dev)
 {
 	struct uart_amba_port *uap = amba_get_drvdata(dev);
+	bool busy = false;
 	int i;
 
 	uart_remove_one_port(&amba_reg, &uap->port);
@@ -2183,9 +2184,12 @@ static int pl011_remove(struct amba_device *dev)
 	for (i = 0; i < ARRAY_SIZE(amba_ports); i++)
 		if (amba_ports[i] == uap)
 			amba_ports[i] = NULL;
+		else if (amba_ports[i])
+			busy = true;
 
 	pl011_dma_remove(uap);
-	uart_unregister_driver(&amba_reg);
+	if (!busy)
+		uart_unregister_driver(&amba_reg);
 	return 0;
 }