ASoC: soc-core: set component->debugfs_root NULL

To be more safety code, let's set NULL to component->debugfs_root
when it was cleanuped.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87muglahq0.wl-kuninori.morimoto.gx@renesas.com


Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>