pwm: Make pwm_apply_state_debug() static

Fix the following gcc warning:

    drivers/pwm/core.c:467:6: warning: symbol 'pwm_apply_state_debug' was
        not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/core.c

@@ -464,7 +464,7 @@ void pwm_free(struct pwm_device *pwm)
 }
 EXPORT_SYMBOL_GPL(pwm_free);
 
-void pwm_apply_state_debug(struct pwm_device *pwm,
+static void pwm_apply_state_debug(struct pwm_device *pwm,
 				  const struct pwm_state *state)
 {
 	struct pwm_state *last = &pwm->last;