USB: dummy-hcd: don't set gadget.speed in dummy_hub_control()

There will never be a case when gadget.speed isn't already
USB_SPEED_FULL if connection is not USB-3 and gadget.speed
is not USB_SPEED_HIGH or USB_SPEED_LOW.

Remove the unnecessary code.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>