leds: trig-netdev: Disable offload on deactivation of trigger

Ensure that the offloading of blinking is stopped when the trigger is
deactivated. Calling led_set_brightness() is documented as stopping
offload and setting the LED to a constant brightness.

Suggested-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Link: https://lore.kernel.org/r/20230808210436.838995-5-andrew@lunn.ch


Signed-off-by: Jakub Kicinski <kuba@kernel.org>