backlight: lm3630a_bl: Stop messing with the pwm->period field

pwm->period field is not supposed to be changed by PWM users. The only
ones authorized to change it are the PWM core and PWM drivers.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>