cfg80211: average ack rssi support for data frames

Average ack rssi will be given to userspace via NL80211 interface
if firmware is capable. Userspace tool ‘iw’ can process this
information and give the output as one of the fields in
‘iw dev wlanX station dump’.

Example output :

localhost ~ #iw dev wlan-5000mhz station dump Station
34:f3:9a:aa:3b:29 (on wlan-5000mhz)
        inactive time:  5370 ms
        rx bytes:       85321
        rx packets:     576
        tx bytes:       14225
        tx packets:     71
        tx retries:     0
        tx failed:      2
        beacon loss:    0
        rx drop misc:   0
        signal:         -54 dBm
        signal avg:     -53 dBm
        tx bitrate:     866.7 MBit/s VHT-MCS 9 80MHz short GI VHT-NSS 2
        rx bitrate:     866.7 MBit/s VHT-MCS 9 80MHz short GI VHT-NSS 2
        avg ack signal: -56 dBm
        authorized:     yes
        authenticated:  yes
        associated:     yes
        preamble:       short
        WMM/WME:        yes
        MFP:            no
        TDLS peer:      no
        DTIM period:    2
        beacon interval:100
       short preamble: yes
       short slot time:yes
       connected time: 203 seconds

Main use case is to measure the signal strength of a connected station
to AP. Data packet transmit rates and bandwidth used by station can vary
a lot even if the station is at fixed location, especially if the rates
used are multi stream(2stream, 3stream) rates with different bandwidth(20/40/80 Mhz).
These multi stream rates are sensitive and station can use different transmit power
for each of the rate and bandwidth combinations. RSSI measured from these RX packets
on AP will be not stable and can vary a lot with in a short time.
Whereas 802.11 ack frames from station are sent relatively at a constant
rate (6/12/24 Mbps) with constant bandwidth(20 Mhz).
So average rssi of the ack packets is good and more accurate.

Signed-off-by: Balaji Pothunoori <bpothuno@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>