gpio: mvebu/pwm: Refuse requests with inverted polarity

The driver doesn't take struct pwm_state::polarity into account when
configuring the hardware, so refuse requests for inverted polarity.

Fixes: 757642f9

 ("gpio: mvebu: Add limited PWM support")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>