mac80211: add a flag to indicate CCK support for HT clients

brcm80211 cannot handle sending frames with CCK rates as part of an
A-MPDU session. Other drivers may have issues too. Set the flag in all
drivers that have been tested with CCK rates.

This fixes a reported brcmsmac regression introduced in
commit ef47a5e4f1aaf1d0e2e6875e34b2c9595897bef6
"mac80211/minstrel_ht: fix cck rate sampling"

Cc: stable@vger.kernel.org # 3.10
Reported-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>