wifi: mt76: mt7921: drop TXRX_NOTIFY on non-mmio buses

PKT_TYPE_TXRX_NOTIFY is an mmio-only event, but mt7921_rx_check() and
mt7921_queue_rx_skb() dispatch it to mt7921_mac_tx_free() on every bus.
mt7921_mac_tx_free() cleans the DMA tx queues with
mt76_queue_tx_cleanup(), which calls queue_ops->tx_cleanup(). Only the
mmio queue ops implement that callback; on USB and SDIO it is NULL, so
a TXRX_NOTIFY there calls a NULL pointer in the RX worker:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  RIP: 0010:0x0
  Call Trace:
   mt7921_mac_tx_free+0x64/0x310 [mt7921_common]
   mt7921_rx_check+0x5f/0xf0 [mt7921_common]
   mt76u_rx_worker+0x1b9/0x620 [mt76_usb]

Drop the event on non-mmio buses via mt76_is_mmio(), as in
commit 5683e1488aa9 ("wifi: mt76: connac: do not check WED status for
non-mmio devices").

Fixes: 48fab5bbef40 ("mt76: mt7921: introduce mt7921s support")
Cc: stable@vger.kernel.org
Signed-off-by: Devin Wittmayer <lucid_duck@justthetip.ca>
Link: https://patch.msgid.link/20260627191336.20223-2-lucid_duck@justthetip.ca
Signed-off-by: Felix Fietkau <nbd@nbd.name>

Author: Lucid-Duck

mt7921/mac.c

@@ -570,8 +570,9 @@ bool mt7921_rx_check(struct mt76_dev *mdev, void *data, int len)
 
 	switch (type) {
 	case PKT_TYPE_TXRX_NOTIFY:
-		/* PKT_TYPE_TXRX_NOTIFY can be received only by mmio devices */
-		mt7921_mac_tx_free(dev, data, len); /* mmio */
+		if (!mt76_is_mmio(mdev))
+			return false;
+		mt7921_mac_tx_free(dev, data, len);
 		return false;
 	case PKT_TYPE_TXS:
 		for (rxd += 2; rxd + 8 <= end; rxd += 8)
@@ -600,7 +601,10 @@ void mt7921_queue_rx_skb(struct mt76_dev *mdev, enum mt76_rxq_id q,
 
 	switch (type) {
 	case PKT_TYPE_TXRX_NOTIFY:
-		/* PKT_TYPE_TXRX_NOTIFY can be received only by mmio devices */
+		if (!mt76_is_mmio(mdev)) {
+			napi_consume_skb(skb, 1);
+			break;
+		}
 		mt7921_mac_tx_free(dev, skb->data, skb->len);
 		napi_consume_skb(skb, 1);
 		break;