pwm: lpc18xx-sct: Don't modify the cached period of other PWM outputs

Even though the hardware only supports a single period for all PWM
outputs, modifying the other (disabled) outputs's period is strange and
wrong. Only the pwm core is supposed to update these values.

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