ASoC: Intel: Skylake: Fix NULL Pointer exception in dynamic_debug.

The following bug was reported by sometime back:

https://lkml.org/lkml/2016/6/29/795



This commit fixes this bug by setting value for the prefix string.

Signed-off-by: Vedang Patel <vedang.patel@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>