usb: usbtest: support usb2 extension descriptor test

In Test 9 of usbtest module, it is used for performing chapter 9 tests N
times.

USB2.0 Extension descriptor is one of the generic device-level capbility
descriptors which added in section 9.6.2.1 of USB 3.0 spec.

This patch adds to support getting usb2.0 extension descriptor test
scenario for USB 3.0.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>