cfg80211: fix can_beacon_sec_chan, reenable HT40

This follows wireless-testing 9236d838
("cfg80211: fix extension channel checks to initiate communication") and
fixes accidental case fall-through. Without this fix, HT40 is entirely
blocked.

Signed-off-by: Mark Mentovai <mark@moxienet.com>
Cc: stable@kernel.org
Acked-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>