usb: gadget: Update config for SuperSpeedPlus

When a function is added to a configuration with usb_add_function(), the
configuration speed flags are updated. These flags indicate for which
speeds the configuration is valid for.

This patch adds a flag in the configuration for SuperSpeedPlus and
also updates this based on the existence of ssp_descriptors.

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