ASoC: arizona: Exit startup early if no runtime

commit 9b8ef9f6 ("ASoC: dapm: Add startup & shutdown for dai_links")

Added support for calling startup on CODEC to CODEC links, however this
is called with a NULL runtime pointer. There isn't really a sensible way
to pass a valid runtime pointer to a CODEC to CODEC link at the moment,
so we need to make the startup function safe for NULL runtimes.

This patch returns from the Arizona startup function early if there is no
runtime, this is perfectly safe as all the startup function does is set
the PCM constraints for user-space which arn't relevant to a CODEC to
CODEC link anyway.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>