serial: mvebu-uart: free the IRQ in ->shutdown()

As suggested by the serial port infrastructure documentation, the IRQ is
requested in ->startup(). However, it is never freed in the ->shutdown()
hook.

With simple systems that open the serial port once for all and always
have at least one process that keep the serial port opened, there was no
problem. But with a more complicated system (*cough* systemd *cough*),
the serial port is opened/closed many times, which at some point no
processes having the serial port open at all. Due to this ->startup()
gets called again, tries to request_irq() again, which fails.

Fixes: 30530791a7 ("serial: mvebu-uart: initial support for Armada-3700 serial port")
Cc: <stable@vger.kernel.org>
Cc: Ofer Heifetz <oferh@marvell.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/mvebu-uart.c

@@ -300,6 +300,8 @@ static int mvebu_uart_startup(struct uart_port *port)
 static void mvebu_uart_shutdown(struct uart_port *port)
 {
 	writel(0, port->membase + UART_CTRL);
+
+	free_irq(port->irq, port);
 }
 
 static void mvebu_uart_set_termios(struct uart_port *port,