mac80211: fix HT mode selection for ad-hoc

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

Backport of r43769

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@43770 3c298f89-4303-0410-b956-a3cf2f4a3e73