mac80211: add throughput based LED blink trigger

iwlwifi and other drivers like to blink their LED
based on throughput. Implement this generically in
mac80211, based on a throughput table the driver
specifies. That way, drivers can set the blink
frequencies depending on their desired behaviour
and max throughput.

All the drivers need to do is provide an LED class
device, best with blink hardware offload.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>