leds: pwm: simplify if condition

.pwm_period_ns is an unsigned integer. So when led->pwm_period_ns > 0
is false, we now assign 0 to a value that is already 0, so it doesn't
hurt and we can skip checking the actual value.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Tested-by: Jeff LaBundy <jeff@labundy.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>