regmap: set debugfs_name to NULL after it is freed

There is a upstream commit cffa4b21("regmap:debugfs:
Fix a memory leak when calling regmap_attach_dev") that
adds a if condition when create name for debugfs_name.
With below function invoking logical, debugfs_name is
freed in regmap_debugfs_exit(), but it is not created again
because of the if condition introduced by above commit.
regmap_reinit_cache()
	regmap_debugfs_exit()
	...
	regmap_debugfs_init()
So, set debugfs_name to NULL after it is freed.

Fixes: cffa4b21 ("regmap: debugfs: Fix a memory leak when calling regmap_attach_dev")
Signed-off-by: Meng Li <Meng.Li@windriver.com>
Link: https://lore.kernel.org/r/20210226021737.7690-1-Meng.Li@windriver.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regmap-debugfs.c

@@ -660,6 +660,7 @@ void regmap_debugfs_exit(struct regmap *map)
 		regmap_debugfs_free_dump_cache(map);
 		mutex_unlock(&map->cache_lock);
 		kfree(map->debugfs_name);
+		map->debugfs_name = NULL;
 	} else {
 		struct regmap_debugfs_node *node, *tmp;