hwmon: pwm-fan: Use 64-bit division macro

Since the PWM framework is switching struct pwm_args.period's datatype
to u64, prepare for this transition by using DIV_ROUND_UP_ULL to handle
a 64-bit dividend.

Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>