USB: dummy-hcd: Adapt dummy_udc_set_speed()

The UDC core ensures that .udc_set_speed() is called with
a speed that is a minimum of the max speeds supported
by the gadget function driver and the UDC driver.

We can now use the speed argument as is.

Get rid of the debug print as that condition will never happen.

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>