Fix tud_video_n_streaming check

[HiFiPhile]

Describe the PR
I met a racing condition with video examples (strangely I didn't caught it in #2538).

With ISO alloc stm->desc.ep[0] is no longer cleared, tud_video_n_streaming() return true when interface is initializing but usbd_edpt_iso_activate() is not yet called. Resulting usbd_edpt_xfer() called before endpoint is ready.

[hathach]

thank you, look good to me. Note: host can acutally do probe while streaming to dynamic change streaming parameter. So I guess the current state does not accurately reflect the streaming state. Though we will do a full revise later on.