pwm-sun4i: Convert "next_period" to local variable

Its value is calculated in sun4i_pwm_apply() and is used only there.

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