ASoC: SOF: ipc-msg-injector: Use devm_kzalloc() for the rx_buffer

ASoC: SOF: ipc-msg-injector: Use devm_kzalloc() for the rx_buffer

The rx_buffer is cleared before sending an IPC to make sure that when the
/sys/kernel/debug/sof/ipc_msg_inject file is read we will have correct
information in the buffer (no random or stale data).

But if the user reads the file before sending any message the buffer might
contain garbage which should not be interpreted.

To prevent this, clear the rx_buffer on allocation.

Fixes: fe7be92b8b75 ("ASoC: SOF: Convert the generic IPC message injector into SOF client")
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20220309110104.18370-1-peter.ujfalusi@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>