ASoC: soc-core: setup card->rtd_num at snd_soc_instantiate_card()

card->rtd_num is used to count rtd. Initialize it at
snd_soc_instantiate_card() is very natural and less confusion.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87zhijafgk.wl-kuninori.morimoto.gx@renesas.com


Signed-off-by: Mark Brown <broonie@kernel.org>