usb: emul: hub: Report the actual device speed of the emulation device

At present the usb hub emulator always reports its downstream port
speed as full speed. Actually it is high speed for sandbox-flash,
and low speed for sandbox-keyb. We can determine the device speed
by checking its device descriptor bcdUSB field, and do the proper
hub port status report based on that.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>