leds: trigger: Store brightness set by led_trigger_event()

If a simple trigger is assigned to a LED, then the LED may be off until
the next led_trigger_event() call. This may be an issue for simple
triggers with rare led_trigger_event() calls, e.g. power supply
charging indicators (drivers/power/supply/power_supply_leds.c).
Therefore persist the brightness value of the last led_trigger_event()
call and use this value if the trigger is assigned to a LED.
In addition add a getter for the trigger brightness value.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/b1358b25-3f30-458d-8240-5705ae007a8a@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>