ASoC: remove rtd->platform checck

Now, we are ready to replace rtd->platform to rtdcom list.
From this patch, rtd->platform check is no longer needed.
It will be replaced into rtdcom.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>