rockchip: Setup default PWM flags

At present if the

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 874ee59 (rockchip: pwm: implement pwm_set_invert())

drivers/pwm/rk_pwm.c

@@ -92,6 +92,7 @@ static int rk_pwm_probe(struct udevice *dev)
 		return -EINVAL;
 	}
 	priv->freq = clk_get_rate(&clk);
+	priv->enable_conf = PWM_DUTY_POSTIVE | PWM_INACTIVE_POSTIVE;
 
 	return 0;
 }