mfd: wm8400-core: Supply description for wm8400_reset_codec_reg_cache's arg

Kerneldoc syntax is used, but not complete.  Descriptions required.

Prevents warnings like:

 drivers/mfd/wm8400-core.c:113: warning: Function parameter or member 'wm8400' not described in 'wm8400_reset_codec_reg_cache'

Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: patches@opensource.cirrus.com
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/wm8400-core.c

@@ -108,6 +108,8 @@ static const struct regmap_config wm8400_regmap_config = {
 /**
  * wm8400_reset_codec_reg_cache - Reset cached codec registers to
  * their default values.
+ *
+ * @wm8400: pointer to local driver data structure
  */
 void wm8400_reset_codec_reg_cache(struct wm8400 *wm8400)
 {