ASoC: omap: Use WARN_ON() instead of BUG_ON()

Use WARN_ON() and handle the error cases accordingly.

Acked-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@linaro.org>