drivers: pwm: pwm_pca9685 set_pre_scale when not in restart mode

The pca9685 driver assumes the chip will be in "restart" mode
after putting it to sleep. This is not necessarily the case, which
can cause setting the prescaler to fail.
This fix allows the precaler to always be set and only restart the pwm's
when the chip was actually in restart mode after being put to sleep.

Signed-off-by: Jaap Versteegh <j.r.versteegh@gmail.com>