leds: pwm: convert to atomic PWM API

pwm_config(), pwm_enable() and pwm_disable() should get removed in the
long run. So update the driver to use the atomic API that is here to
stay.

A few side effects:

 - led_pwm_set() now returns an error when setting the PWM fails.
 - During .probe() the PWM isn't disabled implicitly by pwm_apply_args()
   any more.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Tested-by: Jeff LaBundy <jeff@labundy.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>