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>