usb: gadget: dummy_hcd: use less checks for USB_SPEED_SUPER

Taking the correct struct once avoids doing the speed dance.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>