mac80211/minstrel_ht: fix cck rate sampling

The CCK group needs special treatment to set the right flags and rate
index. Add this missing check to prevent setting broken rates for tx
packets.

Cc: stable@vger.kernel.org # 3.10
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>