leds: led-triggers: Improvements for default trigger

leds: led-triggers: Improvements for default trigger

Accept "default" written to sysfs trigger attr.
If the text "default" is written to the LED's sysfs 'trigger' attr, then
call led_trigger_set_default() to set the LED to its default trigger.

If the default trigger is set to "none", then led_trigger_set_default()
will remove a trigger. This is in contrast to the default trigger being
unset, in which case led_trigger_set_default() does nothing.

Signed-off-by: Craig McQueen <craig@mcqueen.au>
Reviewed-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Link: https://lore.kernel.org/r/20250317022630.424015-1-craig@mcqueen.au
Signed-off-by: Lee Jones <lee@kernel.org>