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>