fastboot: Support USB 3.x SuperSpeed devices on Linux

When enumerating USB 3.x devices, an extra 6-byte SuperSpeed
companion descriptor follows each standard endpoint descriptor.
The loop parsing the descriptors will stumble if they're not
handled. Since they're not needed once the bulk endpoints are
identified, we can simply skip over them.

Change-Id: I7e73bcf9135f23c3059cd7c55b432557b28cb6ef