pwm: pca9685: Restrict period change for enabled PWMs

Previously, the last used PWM channel could change the global prescale
setting, even if other channels are already in use.

Fix it by only allowing the first enabled PWM to change the global
chip-wide prescale setting. If there is more than one channel in use,
the prescale settings resulting from the chosen periods must match.

GPIOs do not count as enabled PWMs as they are not using the prescaler
and can't change it.

Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>