pwm: pca9685: Replace CONFIG_PM with __maybe_unused

The __maybe_unused attribute is preferred over CONFIG_PM to prevent
potential build time issues. This commit replaces CONFIG_PM with this
attribute.

Signed-off-by: Rishi Gupta <gupt21@gmail.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>