serdev: Make .remove in struct serdev_device_driver optional

Using devres infrastructure it is possible to write a serdev driver
that doesn't have any code that needs to be called as a part of
.remove. Add code to make .remove optional.

Cc: linux-kernel@vger.kernel.org
Cc: linux-serial@vger.kernel.org
Cc: Rob Herring <robh@kernel.org>
Cc: cphealy@gmail.com
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Johan Hovold <johan@kernel.org>
Cc: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: NRob Herring <robh@kernel.org>
Reviewed-by: NSebastian Reichel <sebastian.reichel@collabora.co.uk>
Reviewed-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NAndrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serdev/core.c

@@ -268,8 +268,8 @@ static int serdev_drv_probe(struct device *dev)
 static int serdev_drv_remove(struct device *dev)
 {
 	const struct serdev_device_driver *sdrv = to_serdev_device_driver(dev->driver);
-
-	sdrv->remove(to_serdev_device(dev));
+	if (sdrv->remove)
+		sdrv->remove(to_serdev_device(dev));
 	return 0;
 }