hwmon: (pwm-fan) stop using legacy PWM functions and some cleanups

pwm_apply_state() does what the legacy functions pwm_config() and
pwm_{en,dis}able() do in a single function call. This simplifies error
handling and is more efficient for new-style PWM hardware drivers.

Instead of repeatedly querying the PWM framework about the initial PWM
configuration, cache the settings in driver data.

Also use __set_pwm() in .probe() to have the algorithm calculating the PWM
state in a single place.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210112191314.124686-1-u.kleine-koenig@pengutronix.de
[groeck: Added missing empty line after declaration]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>