exynos: pwm: Deal with a PWM at 100%

At present the counter never hits the comparitor in this case. Add a
special case.

This ensures that the snow backlight works when at full brightness.

Fixes: 76c2ff3e video: backlight: fix pwm's duty cycle calculation

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>