libusb: implement libusb_hotplug_get_user_data

libusb provides a function to get the callback userdata for a given
callback since this structure is opaque to libusb user.

Approved by: markj (mentor), lwhsu (mentor)
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D51223