Currently, below fixed rate commands are broken,
iw wlanx set bitrates legacy-<2.4|5> ht-mcs-<2.4|5> vht-mcs-<2.4|5> \
<NSS:MCSx>
iw wlanx set bitrates legacy-<2.4|5> <legacy rate> ht-mcs-<2.4|5> \
vht-mcs-<2.4|5> <NSS:MCSx>

There are two methods to set fixed rate, both failed,
- Use vdev fixed rate command
  This command only support one single rate, but it's broken due to
  mac80211 change commit e8e4f528 ("mac80211: reject/clear user
  rate mask if not usable"), which requires user to specify at least
  one legacy rate. So we can't use this command to set ht/vht single
  rate any more.
- Use peer_assoc command
  This command can update rx capability for multiple rates, it will
  work fine for ht mcs rates, as each supported mcs can be advertised
  in ht_mcs index mask. But this will not work with vht rates because,
  as per the vht mcs capability advertisement, there are only two bits
  to indicate the supported mcs. E.g. only support 0-7, 0-8, 0-9.

So introduced new WMI command: WMI_PEER_PARAM_FIXED_RATE. After peer
assoc, the peer fixed rate cmd will work for that specific peer.
Remaining peers will use auto rate. If both vdev fixed rate and peer
fixed rates are given, peer fixed rate will take effect to peers for
which this cmd is given. Remaining peers in that vdev, will use vdev
fixed rate.

Tested HW: QCA9984
Tested FW: 10.4-3.9.0.2-00035
Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>