ASoC: Fix build error in twl4030.c

Fix the (likely cut-n-paste) error by commit
16a30fbb0d3aa4ee829a2dd3d0e314e2b5ae96a9, which causes the error below:
  sound/soc/codecs/twl4030.c: In function 'twl4030_read_reg_cache':
  sound/soc/codecs/twl4030.c:152: error: 'cache' undeclared (first use in this function)

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

sound/soc/codecs/twl4030.c

@@ -145,6 +145,7 @@ struct twl4030_priv {
 static inline unsigned int twl4030_read_reg_cache(struct snd_soc_codec *codec,
 	unsigned int reg)
 {
+	u8 *cache = codec->reg_cache;
 
 	if (reg >= TWL4030_CACHEREGNUM)
 		return -EIO;