Ensure that pwm control cache is current before updating values

Ensure that pwm control cache is current before updating values

If sensor attributes were never read, the pwm control data has not been
initiialized, which can cause wrong driver behavior. Ensure that cached
data is current before acting on it.

Reported-by: Kevin Folz <kfolz@evertz.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>