driver: 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.

Signed-off-by: Meng Li <Meng.Li@windriver.com>
Signed-off-by: Zhantao Tang <zhantao.tang@windriver.com>