wifi: rt2x00: add throughput LED trigger

This adds a (currently missing) throughput LED trigger for the rt2x00
driver. Previously, LED triggers had to be assigned to the netdev, which
was limited to a single VAP.

Tested-by: Christoph Krapp <achterin@googlemail.com>
Signed-off-by: David Bauer <mail@david-bauer.net>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/73f5ba4134e621462a26186449400cf0c1ac1730.1663445157.git.daniel@makrotopia.org