ASoC: Intel: Common: Fix NULL dereference in tx_wait_done

rx_data and rx_bytes present for tx_wait_done are optional parameters.
If not provided, function should not attempt to copy received data.
This change fixes memcpy NULL pointer dereference issue occurring when
optional rx_data is NULL while received message size is non-zero.

Signed-off-by: Cezary Rojewski <cezary.rojewski@intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>