leds: triggers: Don't remove trigger if LED_KEEP_TRIGGER flag is set

Currently, LED_KEEP_TRIGGER flag prevents the trigger being removed
while turning off the LEDs. Extend the flag usage to prevent the trigger
being removed even while "none" trigger is set.

Change-Id: I2f3e7bf07b3191a8224732163b67dbd726b76c3b
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>