serial: serial_core: Perform NULL checks for release/request_port ops

Doing the following UART bind/unbind sequence on a i.mx platform causes
a kernel crash due to NULL pointer dereference:

echo  21f4000.serial > /sys/bus/platform/drivers/imx-uart/bind
echo  21f4000.serial > /sys/bus/platform/drivers/imx-uart/unbind

Fix this problem by adding NULL checks prior to calling release/request_port
ops.
Reported-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Fugang Duan <fugang.duan@nxp.com>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -887,7 +887,7 @@ static int uart_set_info(struct tty_struct *tty, struct tty_port *port,
 		/*
 		 * Free and release old regions
 		 */
-		if (old_type != PORT_UNKNOWN)
+		if (old_type != PORT_UNKNOWN && uport->ops->release_port)
 			uport->ops->release_port(uport);
 
 		uport->iobase = new_port;
@@ -900,7 +900,7 @@ static int uart_set_info(struct tty_struct *tty, struct tty_port *port,
 		/*
 		 * Claim and map the new regions
 		 */
-		if (uport->type != PORT_UNKNOWN) {
+		if (uport->type != PORT_UNKNOWN && uport->ops->request_port) {
 			retval = uport->ops->request_port(uport);
 		} else {
 			/* Always success - Jean II */
@@ -1125,7 +1125,7 @@ static int uart_do_autoconfig(struct tty_struct *tty,struct uart_state *state)
 		 * If we already have a port type configured,
 		 * we must release its resources.
 		 */
-		if (uport->type != PORT_UNKNOWN)
+		if (uport->type != PORT_UNKNOWN && uport->ops->release_port)
 			uport->ops->release_port(uport);
 
 		flags = UART_CONFIG_TYPE;
@@ -2897,7 +2897,7 @@ int uart_remove_one_port(struct uart_driver *drv, struct uart_port *uport)
 	/*
 	 * Free the port IO and memory resources, if any.
 	 */
-	if (uport->type != PORT_UNKNOWN)
+	if (uport->type != PORT_UNKNOWN && uport->ops->release_port)
 		uport->ops->release_port(uport);
 	kfree(uport->tty_groups);