backlight: pwm_bl: Use 64-bit division function

Since the PWM framework is switching struct pwm_state.period's datatype
to u64, prepare for this transition by using div_u64 to handle a 64-bit
dividend instead of a straight division operation.

Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>