mfd: wm8994: Remove unused irq_lock

Since the conversion to regmap-irq irq_lock has been unused.
Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

include/linux/mfd/wm8994/core.h

@@ -56,8 +56,6 @@ struct irq_domain;
 #define WM8994_IRQ_GPIO(x) (x + WM8994_IRQ_TEMP_WARN)
 
 struct wm8994 {
-	struct mutex irq_lock;
-
 	struct wm8994_pdata pdata;
 
 	enum wm8994_type type;