wcn36xx: Implement tx_rate reporting

Currently, the driver reports a tx_rate of 6.0 MBit/s no matter the true
rate:

root@linaro-developer:~# iw wlan0 link
Connected to 6c:f3:7f:eb:9b:92 (on wlan0)
        SSID: SQ-DEVICETEST
        freq: 5200
        RX: 4141 bytes (32 packets)
        TX: 2082 bytes (15 packets)
        signal: -77 dBm
        rx bitrate: 135.0 MBit/s MCS 6 40MHz short GI
        tx bitrate: 6.0 MBit/s

        bss flags:      short-slot-time
        dtim period:    1
        beacon int:     100

This patch requests HAL_GLOBAL_CLASS_A_STATS_INFO via a hal_get_stats
firmware message and reports it via ieee80211_ops::sta_statistics.

root@linaro-developer:~# iw wlan0 link
Connected to 6c:f3:7f:eb:73:b2 (on wlan0)
        SSID: SQ-DEVICETEST
        freq: 5700
        RX: 26788094 bytes (19859 packets)
        TX: 1101376 bytes (12119 packets)
        signal: -75 dBm
        rx bitrate: 135.0 MBit/s MCS 6 40MHz short GI
        tx bitrate: 108.0 MBit/s VHT-MCS 5 40MHz VHT-NSS 1

        bss flags:      short-slot-time
        dtim period:    1
        beacon int:     100

Tested on MSM8939 with WCN3680B running firmware CNSS-PR-2-0-1-2-c1-00083,
and verified by sniffing frames over the air with Wireshark to ensure the
MCS indices match.

Signed-off-by: Edmond Gagnon <egagnon@squareup.com>
Reviewed-by: Benjamin Li <benl@squareup.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220325224212.159690-1-egagnon@squareup.com