usb: Make USB 3.0 roothub have a SS EP comp descriptor.

Make the USB 3.0 roothub registered by the USB core have a SuperSpeed
Endpoint Companion Descriptor after the interrupt endpoint.  All USB 3.0
devices are required to have this, and the USB 3.0 bus specification
(section 10.13.1) says which values the descriptor should have.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>