ASoC: Improve the debugfs hierarchy

Change the way the debugfs entries are created:
If the codec->dev is valid, than use:
debugfs/asoc/{codec->name}.{dev_name(codec->dev)}/

if the codec->dev is NULL:
debugfs/asoc/{codec->name}/

as root for the debugfs entries.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>