usba: usba_process_cv_descr() needs to handle companion descs
Inspired by this report: https://github.com/joyent/smartos-live/issues/694 and seeing that our own Seagate USB3 drive wouldn't attach, I took the DTrace script provided there, and dove in, trying to see why the drive wouldn't attach.
I found out, once I dove in deep, that usba_process_cv_descr() was failing. It was failing because it didn't recognize the last-processed descriptor type. It should recognize it (a new USB3 descriptor type) and act accordingly. Once this failure is addressed, the rest of the USB3 stack should churn along and accept the drive.