leds: leds-pwm: drop one pwm_get_period() call

pwm_get_period() is called twice in case the child parameter is set. I
assume retrieving this parameter once is enough therefore this patch
removes the conditial invocation of pwm_get_period().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Bryan Wu <cooloney@gmail.com>