pwm: rockchip: Set polarity unconditionally in .get_state()

Don't rely on *state being zero initialized and PWM_POLARITY_NORMAL
being zero. So always assign .polarity.

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