ASoC: remove unneeded dai->driver check

On soc_add_dai(), it is checking dai->driver->xxx,
This means, dai->driver is mandatory, never been NULL.
dai->driver check is not needed.
This patch removes it.

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