pwm: pca9685: Reset OFF/ON registers to POR value

During the driver probe, registers are not set to their POR value.

Signed-off-by: Lionel Vitte <lionel.vitte@free.fr>
Acked-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>