bhyve: use speed in hci to allow backend modify it.

The speed of a USB device may be determined dynamically by the backend.
For example, USB device passthrough depends on the internal device
reporting its speed accurately.

To accommodate this, we now obtain the speed from the USB HCI rather
than initializing it statically from the UE structure. If the backend does
not provide a speed, we fall back to the value in the UE structure.

Reviewed by: emaste
Approved by: markj (mentor), lwhsu (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51411