regmap: set debugfs_name to NULL after it is freed

In the Linux kernel, the following vulnerability has been resolved:

regmap: set debugfs_name to NULL after it is freed

There is a upstream commit cffa4b2122f5("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.