usb: gadget: Update function for SuperSpeedPlus

Add a ssp_descriptors member to struct usb_function and handle the
initialization and cleanup of it. This holds the SuperSpeedPlus
descriptors for a function that supports SuperSpeedPlus. This is added
by usb_assign_descriptors().

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>