pwm-sun4i: Calculate "delay_jiffies" directly, eliminate absolute time

Basically this code did "jiffies + period - jiffies", and we can
simply eliminate the "jiffies" time stamp here.

Signed-off-by: Max Kellermann <max.kellermann@gmail.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>