ASoC: soc-core: Remove unneeded dentry member from snd_soc_codec

There is no need to have the *debugfs_reg dentry member as part of
the snd_soc_codec structure as its only usage is inside
soc_init_codec_debugfs().

Use a local dentry variable instead.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>