usb: gadget: composite: Clear reserved fields of SSP Dev Cap

Set the reserved fields of the SuperSpeed Plus Device Capability
descriptor to 0. Otherwise there might be stale data there which will
cause USB CV to fail.

Fixes: f228a8de ("usb: gadget: composite: Return SSP Dev Cap descriptor")
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>