pwm: sunxi: fix off-by-one that prevented PWM to use prescaler bypass

Fix off-by-one that prevented PWM driver to use prescaler bypass.
Without this change prescaler is always enabled.

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