pwm: sprd: Refuse requests with unsupported polarity

The driver only supports normal polarity and so should refuse requests
for inversed polarity.

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