mtd: core: clear out unregistered devices a bit more

This allows an MTD device that has been unregistered to be easily
re-registered later without triggering spurious "already registered"
warnings in mtd_device_parse_register() and add_mtd_device().

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211015185049.3318-1-zev@bewilderbeest.net

drivers/mtd/mtdcore.c

@@ -747,6 +747,9 @@ int del_mtd_device(struct mtd_info *mtd)
 
 		device_unregister(&mtd->dev);
 
+		/* Clear dev so mtd can be safely re-registered later if desired */
+		memset(&mtd->dev, 0, sizeof(mtd->dev));
+
 		idr_remove(&mtd_idr, mtd->index);
 		of_node_put(mtd_get_of_node(mtd));
 
@@ -1018,8 +1021,10 @@ int mtd_device_unregister(struct mtd_info *master)
 {
 	int err;
 
-	if (master->_reboot)
+	if (master->_reboot) {
 		unregister_reboot_notifier(&master->reboot_notifier);
+		memset(&master->reboot_notifier, 0, sizeof(master->reboot_notifier));
+	}
 
 	if (master->otp_user_nvmem)
 		nvmem_unregister(master->otp_user_nvmem);