ASoC: simple-card-utils: accept NULL parameter on asoc_simple_card_xxx()

If simple-card-utils accept NULL pointer on asoc_simple_card_xxx(),
each driver code will be more simple.
Let's accept NULL pointer.

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