ASoC: SOF: trace: remove code duplication in sof_wait_trace_avail()

Move duplicated code in sof_wait_trace_avail() to a helper function.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>