ASoC: topology: Fix memory leak from soc_tplg_denum_create_texts

dtexts is two dimensional array, so we also need to free it after
freeing its fields.

Signed-off-by: Amadeusz Sławiński <amadeuszx.slawinski@intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>