mac80211: make it work with 3.18.12+

3.18.12 backported 61ada528dea028331e99e8ceaed87c683ad25de2 ("sched/wait:
Provide infrastructure to deal with nested blocking") from 3.19, causing
the following error on load:

[   13.588000] compat: exports duplicate symbol woken_wake_function (owned by kernel)

Fix this by guarding it with a check for 3.18.11 or earlier instead of
3.19.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45710 3c298f89-4303-0410-b956-a3cf2f4a3e73