ASoC: meson: axg-tdmin: remove useless assignment

cppcheck complains about potential null pointer dereference but it's
rather an unnecessary assignment to NULL before walking through a
list.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210326215927.936377-6-pierre-louis.bossart@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>