mfd: wm831x-core: Fix incorrect function name wm831x_reg_unlock()

Fixes the following W=1 kernel build warning(s):

 drivers/mfd/wm831x-core.c:121: warning: expecting prototype for wm831x_reg_unlock(). Prototype was for wm831x_reg_lock() instead

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