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>

drivers/mfd/wm831x-core.c

@@ -109,7 +109,7 @@ static int wm831x_reg_locked(struct wm831x *wm831x, unsigned short reg)
 }
 
 /**
- * wm831x_reg_unlock: Unlock user keyed registers
+ * wm831x_reg_lock: Unlock user keyed registers
  *
  * The WM831x has a user key preventing writes to particularly
  * critical registers.  This function locks those registers,