usb: cdns3: gadget: Fix g_audio use case when connected to Super-Speed host

Take into account gadget driver's speed limit when programming
controller speed.

Fixes: 7733f6c3

 ("usb: cdns3: Add Cadence USB3 DRD Driver")
Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Peter Chen <peter.chen@nxp.com>
Link: https://lore.kernel.org/r/20191030121607.21739-1-rogerq@ti.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>