mac80211: selective throughput LED trigger active

The throughput LED trigger was always active when
the radio was enabled. In most cases that's likely
the desired behaviour, but iwlwifi requires it to
be only active when one of the virtual interfaces
is actually "connected" in some way.

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