mac80211: Remove WARN_ON in apply-ht-override logic.

AP interfaces routinely call this logic, so just silently
return when this happens instead of splatting the kernel
logs.

Reported-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>