wifi: mt76: mt76u: use a threaded NAPI for the RX path

The USB RX path delivers frames to the stack via mt76_rx_complete() with
a NULL napi pointer, taking the netif_receive_skb_list() path, so it never
benefits from GRO -- unlike the DMA-based mt76 drivers, which pass a real
napi and use napi_gro_receive(). For bulk TCP traffic this is costly, as
every segment traverses the stack individually.

Service the MT_RXQ_MAIN queue from a threaded NAPI, reusing mt76_dev's
existing napi_dev and napi[] rather than adding new fields. The URB
completion handler schedules the napi; its poll drains the URBs, builds
the skbs, resubmits and delivers them through napi_gro_receive(). The MCU
queue stays on the existing RX worker. This enables GRO and moves RX
processing into its own kernel thread, parallelising the datapath.

On mt7921u at HE-MCS 11 (2x2, 80 MHz; fast.com, multiple streams) this
averages ~588 Mbit/s, versus ~424 Mbit/s when the same napi is instead
driven manually from the RX worker, and ~380 Mbit/s for the unmodified
driver.

Suggested-by: Lorenzo Bianconi <lorenzo@kernel.org>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Filip Bakreski <phial@phiality.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260609105301.196302-1-phial@phiality.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>

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