dcd/musb: name pipe0_state_t, use local pointer, group struct fields

Pure cleanup, no behavior change:
- Extract the EP0 control-transfer state into a named pipe0_state_t
  typedef instead of an anonymous nested struct, and access it through
  a local pipe0_state_t* in the functions that touch it repeatedly.
- Group the pipe0 fields so the two bools sit together and the larger
  tusb_control_request_t deferred_setup is last.
- Reword the deferral comments: "coalesced" -> "combined".

Note: separating the edpt0_xfer DATA_IN/DATA_OUT case (dispatch on
state instead of dir_in) was attempted and reverted — it breaks ADI
MUSB enumeration. usbd can arm the opposite-direction status while
pipe0 is still in a DATA state, and only dir-dispatch routes that
correctly; a comment on the combined case records this.

Verified: HIL pass on ek_tm4c123gxl and max32666fthr (13/13 each).

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

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