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.

Change-Id: Ia54fce85b570715e705f67bd463c4a219ff71af2
CRs-fixed: 1116687
Signed-off-by: Thomas Pedersen <twp@qca.qualcomm.com>
Cc: Johannes Berg <johannes.berg@intel.com>
Cc: greearb@candelatech.com
Cc: Thomas Pedersen <twp@qca.qualcomm.com>
Patch-mainline: linux-wireless @ 09/10/16, 23:35
Signed-off-by: Peng Xu <pxu@codeaurora.org>