rpmsg: core: Make remove handler for rpmsg driver optional.

Most other bus (for example, SPI, i2c) have the remove handler for
driver optional. Make remove handler for rpmsg driver optional too.

Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

drivers/rpmsg/rpmsg_core.c

@@ -493,7 +493,8 @@ static int rpmsg_dev_remove(struct device *dev)
 	if (rpdev->ops->announce_destroy)
 		err = rpdev->ops->announce_destroy(rpdev);
 
-	rpdrv->remove(rpdev);
+	if (rpdrv->remove)
+		rpdrv->remove(rpdev);
 
 	dev_pm_domain_detach(dev, true);