pwm: Fix device reference leak

Make sure to drop the reference to the parent device taken by
class_find_device() after "unexporting" any children when deregistering
a PWM chip.

Fixes: 0733424c

 ("pwm: Unexport children before chip removal")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>