video: backlight: fix pwm inversion

set_pwm() will always fail with -ENOSYS if pwm_ops set_invert() is
not implemented, leaving the backlight dark. Fix this by returning
no error if set_invert() is not implemented and no polarity change
is requested.

Fixes: 57e7775413 ("video: backlight: Parse PWM polarity cell")
Signed-off-by: Marc Dietrich <marvin24@gmx.de>
diff --git a/drivers/video/pwm_backlight.c b/drivers/video/pwm_backlight.c
index a587977..ad20bf2 100644
--- a/drivers/video/pwm_backlight.c
+++ b/drivers/video/pwm_backlight.c
@@ -67,6 +67,9 @@
 		return log_ret(ret);
 
 	ret = pwm_set_invert(priv->pwm, priv->channel, priv->polarity);
+	if (ret == -ENOSYS && !priv->polarity)
+		ret = 0;
+
 	return log_ret(ret);
 }