dcd_lpc17_40: address review findings in the iso paths

From a second max-effort review of the branch:

- Drop the dead TUSB_XFER_ISOCHRONOUS case in dcd_edpt_open: iso endpoints
  are armed via dcd_edpt_iso_alloc/activate (TUP_DCD_EDPT_ISO_ALLOC is
  defined for this IP), never through dcd_edpt_open, so the case and its
  dd->isochronous assignment were unreachable and asserted a false
  invariant. Only bulk/interrupt reach the switch now.

- Extend the iso compile gate to the classes that actually arm an iso
  endpoint: DCD_ISO_ENABLED now includes CFG_TUD_BTH (bth_device.c opens
  an iso voice endpoint). Without it a BTH build would compile the iso
  machinery out and fail SET_INTERFACE at runtime.

- Un-skip LPC175X_6X in the usbtest example: it shares dcd_lpc17_40.c with
  LPC40XX verbatim, so the "DCD has no isochronous support" skip reason no
  longer holds. Build-verified for lpcxpresso1769 (previously blocked by
  the skip).

- TU_ATTR_UNUSED on the ep_id_is_iso helper: every caller is under
  #if DCD_ISO_ENABLED, so non-iso builds don't reference it and clang's
  -Wunused-function (fatal in CI) rejected the build — gcc stays quiet.
  Verified with the full lpc17 and lpc40 example sets under arm-clang.

A fifth finding — bounding control_ep_read's PACKET_READY spin with a
timeout — was implemented and REVERTED: a naive 100k-iteration bound fires
on legitimately-slow control reads and intermittently drops the device
(hardware-proven by interleaved A/B testing against the pre-fix binary).
The infinite wait is retained; the read is only reached once out_received/
out_queued signal data is present, so the theoretical IRQ-off hang is not
reachable in practice.

Re-verified on ea4088_quickstart: usbtest 30/30 (repeated) + HIL 14/14.

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