Add the missing unlock before return from function
snd_cs4231_playback_prepare() in the error handling
case.

Fixes: 5a19b178 ('ALSA: sparc/cs4231: Use WARN_ON() instead of BUG_ON()')
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Takashi Iwai <tiwai@suse.de>