dcd/musb: extract pipe0_read_setup() helper

The 8-byte EP0 SETUP drain (count0 assert + two FIFO word reads via a
union) was duplicated verbatim between the IDLE case and the deferral
case; a future fix applied to one copy but not the other would only
show up on the rare deferred-race path. Share one helper. count0 is
now read inside the only remaining user (DATA OUT drain).

Review follow-up for #3643 (dcd_musb.c l.507 finding).

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

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