backports: use old led api on old kernel versions.

Usage of a new led api was introduced in mac80211, this patch make
backports use the old api on older kernel versions. This could cause a
problem with the led, the transmit led could stay on if nothing is
transfered.

This backports the following upstream commit:
commit e47f2509e5f182f4df144406de6f2bc78179d57e
Author: Fabio Baltieri <fabio.baltieri@gmail.com>
Date:   Thu Jul 25 12:00:26 2013 +0200

    mac80211: use oneshot blink API for LED triggers