pwm: Update comment on struct pwm_ops::apply

Commit 71523d18 (pwm: Ensure pwm_apply_state() doesn't modify the
state argument) updated the kernel-doc for pwm_apply_state(), but not
for the ->apply callback in the pwm_ops struct.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>