video: assert usbd_edpt_iso_activate() result in _open_vs_itf()

The isochronous streaming endpoint was activated with the
usbd_edpt_iso_activate() return value ignored, unlike every neighbouring
open in the same function (usbd_edpt_open() is wrapped in TU_ASSERT on both
the non-ISO-alloc fallback and the bulk branch).

When a DCD refuses the iso endpoint -- e.g. it has no isochronous support, or
the requested packet size does not fit its endpoint buffers -- that failure
was silently swallowed and the alternate setting was reported as opened,
leaving the host streaming to an endpoint the device never armed.

Wrap it in TU_ASSERT so the open fails cleanly and the refusal propagates,
matching the adjacent endpoint-open calls.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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