usb: usbtest: update bos test coverage to usb 2.1 device

The commit "usb: usbtest: support bos descriptor test for usb 3.0"
introduced a test for bos descriptor. And USB 2.1 device also can be
checked. So this patch extends the test coverage to support USB 2.1 device.

Reported-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>