kernel: add a ssb backport required for a new mac80211 update

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r40833

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@40835 3c298f89-4303-0410-b956-a3cf2f4a3e73
Merge remote-tracking branch 'upstream/master' into 720