ASoC: wm8900: Replace BUG() with WARN()

BUG() used in the driver is just to spit the stack trace on buggy
points, not really needed to stop the whole operation.  For that
purpose, it'd be more convenient to use WARN() instead with more
error information.

Cc: patches@opensource.wolfsonmicro.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@linaro.org>