ALSA: pxa2xx: Replace BUG() with snd_BUG()

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 snd_BUG() instead.

Signed-off-by: Takashi Iwai <tiwai@suse.de>