dcd/musb: end EP0 IN data stage on short packet, split DATA case

A short IN control response (device sends fewer bytes than wLength —
e.g. the 18-byte device descriptor answering a 64-byte GET_DESCRIPTOR)
left remain_wlength != 0, so the DATA_IN -> STATUS_OUT transition never
fired and pipe0 stayed in DATA_IN through the status stage. usbd then
armed the status-OUT while state was still DATA_IN. Set DATAEND and
transition on the last packet: remain_wlength == 0, or a short packet
(incl. a terminating ZLP) which ends the data stage.

With state now tracking the stage, split edpt0_xfer's DATA handling
into separate DATA_IN / DATA_OUT cases dispatching on state (asserting
state == call direction) instead of the combined dir_in branch.

Verified: HIL pass on ek_tm4c123gxl and max32666fthr (13/13 each),
including the #3643 high-CPU-load IRQ-toggle coalescing stress.

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

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