wifi: mt76: restrict NPU/PPE active checks to MMIO devices

wifi: mt76: restrict NPU/PPE active checks to MMIO devices

mt76_npu_device_active() and mt76_ppe_device_active() read dev->mmio.npu
and dev->mmio.ppe_dev. The mmio, usb and sdio bus structs share a union in
struct mt76_dev, so on USB and SDIO these read unrelated data from the
usb/sdio struct, which is non-NULL in practice.

mt76_npu_device_active() then returns true on USB, and
mt76_rx_poll_complete() takes the offload path and skips
mt76_rx_aggr_reorder(). RX A-MPDU subframes are delivered out of order and
the peer's TCP stack treats that as loss: heavy retransmissions and reduced
throughput in AP mode. Seen on mt7921u, mt7925u, mt76x2u and mt76x0u.

Gate both helpers on mt76_is_mmio() so they only run for the bus type that
owns the mmio union member.

Fixes: 7fb554b1b623 ("wifi: mt76: Introduce the NPU generic layer")
Cc: stable@vger.kernel.org
Tested-by: Nick Morrow <morrownr@gmail.com>
Signed-off-by: Devin Wittmayer <lucid_duck@justthetip.ca>
Link: https://patch.msgid.link/20260720232640.41293-1-lucid_duck@justthetip.ca
Signed-off-by: Felix Fietkau <nbd@nbd.name>