pwm: rockchip: Unprepare clocks only after the PWM was unregistered

The driver is supposed to stay functional until pwmchip_remove()
returns. So disable clocks only after that.

pwmchip_remove() always returns 0, so the return code can be ignored
which keeps rockchip_pwm_remove() a bit simpler and allows to eventually
make pwmchip_remove() return void.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>