pwm: sprd: Explicitly set .polarity in .get_state()

The driver only supports normal polarity. Complete the implementation of
.get_state() by setting .polarity accordingly.

Fixes: 8aae4b02

 ("pwm: sprd: Add Spreadtrum PWM support")
Link: https://lore.kernel.org/r/20230228135508.1798428-5-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>