cfg80211: add bitrate for 20MHz MCS 9

Some drivers (ath10k) report MCS 9 @ 20MHz, which
technically isn't defined. To get more meaningful value
than 0 out of this however, just extrapolate a bitrate
from ratio of MCS 7 and 9 in channels where it is allowed.
Signed-off-by: Thomas Pedersen <twp@qca.qualcomm.com>
[add a comment about it in the code]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/util.c

@@ -1158,7 +1158,8 @@ static u32 cfg80211_calculate_bitrate_vht(struct rate_info *rate)
 		   58500000,
 		   65000000,
 		   78000000,
-		   0,
+		/* not in the spec, but some devices use this: */
+		   86500000,
 		},
 		{  13500000,
 		   27000000,