wifi: mt76: mt7925: drop TXRX_NOTIFY on non-mmio buses

wifi: mt76: mt7925: drop TXRX_NOTIFY on non-mmio buses

PKT_TYPE_TXRX_NOTIFY is an mmio-only event, but mt7925_rx_check() and
mt7925_queue_rx_skb() dispatch it to mt7925_mac_tx_free() on every bus.
mt7925_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 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:
   mt7925_mac_tx_free+0x58/0x350 [mt7925_common]
   mt7925_rx_check+0xe2/0x130 [mt7925_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: c948b5da6bbe ("wifi: mt76: mt7925: add Mediatek Wi-Fi7 driver for mt7925 chips")
Cc: stable@vger.kernel.org
Signed-off-by: Devin Wittmayer <lucid_duck@justthetip.ca>
Link: https://patch.msgid.link/20260627191336.20223-3-lucid_duck@justthetip.ca
Signed-off-by: Felix Fietkau <nbd@nbd.name>