ASoC: soc-core: Fix null pointer dereference

In case the codec driver did not provide a read/write function,
codec->driver->read|write will be NULL.  Ensure that we use the one
specified in codec->read|write to avoid oopsing when we access
the debugfs entries.  This is achieved by using snd_soc_read() and
snd_soc_write().
Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

sound/soc/soc-core.c

@@ -117,7 +117,7 @@ static ssize_t soc_codec_reg_show(struct snd_soc_codec *codec, char *buf)
 			 * the register being volatile and the device being
 			 * powered off.
 			 */
-			ret = codec->driver->read(codec, i);
+			ret = snd_soc_read(codec, i);
 			if (ret >= 0)
 				count += snprintf(buf + count,
 						  PAGE_SIZE - count,
@@ -228,7 +228,7 @@ static ssize_t codec_reg_write_file(struct file *file,
 		start++;
 	if (strict_strtoul(start, 16, &value))
 		return -EINVAL;
-	codec->driver->write(codec, reg, value);
+	snd_soc_write(codec, reg, value);
 	return buf_size;
 }