ASoC: soc-core: reuse rtdcom at snd_soc_rtdcom_add()

snd_soc_rtdcom_add() is using both "rtdcom" and "new_rtdcom" as
variable name, but these are not used at same time.
Let's reuse rtdcom.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87h86tahp2.wl-kuninori.morimoto.gx@renesas.com
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>