ASoC: Do not write to invalid registers on the wm9712.

commit 48e3cbb3 upstream.

This patch fixes a bug where "virtual" registers were being written to the ac97
bus.  This was causing unrelated registers to become corrupted (headphone 0x04,
touchscreen 0x78, etc).

This patch duplicates protection that was included in the wm9713 driver.
Signed-off-by: Eric Millbrandt <emillbrandt@dekaresearch.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

sound/soc/codecs/wm9712.c

@@ -464,7 +464,8 @@ static int ac97_write(struct snd_soc_codec *codec, unsigned int reg,
 {
 	u16 *cache = codec->reg_cache;
 
-	soc_ac97_ops.write(codec->ac97, reg, val);
+	if (reg < 0x7c)
+		soc_ac97_ops.write(codec->ac97, reg, val);
 	reg = reg >> 1;
 	if (reg < (ARRAY_SIZE(wm9712_reg)))
 		cache[reg] = val;