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

Leaving BUG_ON() in a core layer like dapm is rather inappropriate as
it leads to panic(), even though sanity checks might be still useful
for debugging.
Instead, Use WARN_ON(), and handle the error cases accordingly.

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