pwm: sunxi: use new prescaler when configuring PWM

Looks like old prescaler was used when configuring PWM, fix it.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>