ASoC: davinci: Kill BUG_ON() usage

Don't use BUG_ON() for a non-critical sanity check on production
systems.  This patch replaces with a softer WARN_ON() and an error
path.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>