mfd: wm831x-core: Supply description wm831x_reg_{un}lock args

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

Prevents warnings like:

 drivers/mfd/wm831x-core.c:119: warning: Function parameter or member 'wm831x' not described in 'wm831x_reg_lock'
 drivers/mfd/wm831x-core.c:145: warning: Function parameter or member 'wm831x' not described in 'wm831x_reg_unlock'

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>