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>