mt76: mt7921: add MCU support

[Colin Ian King]

Hi,

Static analysis with Coverity on linux-next has found an issue with the
following commit:

commit 1c099ab44727c8e42fe4de4d91b53cec3ef02860
Author: Sean Wang <sean.wang at mediatek.com>
Date:   Thu Jan 28 03:33:39 2021 +0800

    mt76: mt7921: add MCU support

The analysis is as follows:

390 static void
391 mt7921_mcu_tx_rate_report(struct mt7921_dev *dev, struct sk_buff *skb,
392                          u16 wlan_idx)
393 {
394        struct mt7921_mcu_wlan_info_event *wtbl_info =
395                (struct mt7921_mcu_wlan_info_event *)(skb->data);
396        struct rate_info rate = {};
397        u8 curr_idx = wtbl_info->rate_info.rate_idx;
398        u16 curr = le16_to_cpu(wtbl_info->rate_info.rate[curr_idx]);
399        struct mt7921_mcu_peer_cap peer = wtbl_info->peer_cap;
400        struct mt76_phy *mphy = &dev->mphy;

   1. var_decl: Declaring variable stats without initializer.

401        struct mt7921_sta_stats *stats;
402        struct mt7921_sta *msta;
403        struct mt76_wcid *wcid;
404

   2. Condition wlan_idx >= 288, taking false branch.

405        if (wlan_idx >= MT76_N_WCIDS)
406                return;

   3. Condition 0 /* !((((sizeof ((*dev).mt76.wcid[wlan_idx]) == sizeof
(char) || sizeof ((*dev).mt76.wcid[wlan_idx]) == sizeof (short)) ||
sizeof ((*dev).mt76.wcid[wlan_idx]) == sizeof (int)) || sizeof
((*dev).mt76.wcid[wlan_idx]) == sizeof (long)) || sizeof
((*dev).mt76.wcid[wlan_idx]) == sizeof (long long)) */, taking false branch.

   4. Condition debug_lockdep_rcu_enabled(), taking true branch.
   5. Condition !__warned, taking true branch.
   6. Condition 0, taking false branch.
   7. Condition rcu_read_lock_held(), taking false branch.
407        wcid = rcu_dereference(dev->mt76.wcid[wlan_idx]);
   8. Condition !wcid, taking true branch.
408        if (!wcid) {

Uninitialized pointer write (UNINIT)
   9. uninit_use: Using uninitialized value stats.

409                stats->tx_rate = rate;
410                return;
411        }

Line 409 dereferences pointer stats, however, this pointer has not yet
been initialized.  The initialization occurs later:

413        msta = container_of(wcid, struct mt7921_sta, wcid);
414        stats = &msta->stats;

Colin