mac80211: minstrel_ht: include type (cck/ht) in rates flag

ATM, we grep cck rates idx with idx / MCS_GROUP_RATES ==
MINSTREL_CCK_GROUP.
Matching neither-cck-non-ht rates could be done by replacing '==' with
'>', however it would be less versatile or explicit.
This will allow to match VHT rates with IEEE80211_TX_RC_VHT_MCS.

Signed-off-by: Karl Beldan <karl.beldan@rivierawaves.com>
Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>