pwm: cros-ec: Change prototype of helpers to prepare further changes

pwm: cros-ec: Change prototype of helpers to prepare further changes

pwm_chip allocation and registration is about to change. For that the
number of PWM devices must be known earlier in cros_ec_pwm_probe(). So
make cros_ec_pwm_get_duty() and cros_ec_num_pwms() independent of
struct cros_ec_pwm_device which is only available later.

Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/c0cfc75c1cba0f735555e0138387143ec101feb3.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>