dcd/musb: fix deferred-SETUP replay racing usbd's status call

STATUS_OUT_PENDING conflated "edpt0_xfer(STATUS OUT) called, awaiting
confirm IRQ" with "confirm IRQ seen, awaiting edpt0_xfer". The
deferral path completed the status and replayed the saved SETUP from
the ISR in both flavors; in the IRQ-first one, usbd's still-
outstanding edpt0_xfer(STATUS OUT) for the old transfer (queued via
status_stage_xact) then landed in the replayed transfer's state and
corrupted it: NULL pipe0.buf armed plus RXRDYC, so the host's next
DATA OUT drained through a NULL pointer. usbd processes EP0
XFER_COMPLETE events unconditionally, so nothing downstream defuses
it.

Split the state into STATUS_OUT_PENDING_XFER / _IRQ. The deferral
completes and replays only in PENDING_XFER (old transfer already
retired); in PENDING_IRQ it only holds the SETUP and the usbd-driven
edpt0_xfer fires the completion and replays. The DATA_IN deferral now
synthesizes PENDING_IRQ (its remain==0 invariant asserted: a SETUP
before DataEnd raises SetupEnd instead), which also makes the old
deferred-promotion in the csrl==0 DATA_IN case unreachable - dropped.
Assert the drain buffer before the DATA OUT FIFO read as a cheap
backstop for this corruption class.

Review follow-up for #3643 (dcd_musb.c l.503 finding).

Co-Authored-By: Claude Fable 5 <[email protected]>

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