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.

Cc: stable@vger.kernel.org # 4.4+
Reported-by: NKevin Folz <kfolz@evertz.com>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>