usb: gadget: Update usb_assign_descriptors for SuperSpeedPlus

Add the 'ssp_descriptors' parameter to the usb_assign_descriptors()
function. This allows a function driver to add descriptors for
SuperSpeedPlus speeds if it supports it.

Also update all uses of this function in the gadget subsystem so that
they pass NULL for the ssp_descriptors parameters.

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