usb: gadget: composite: avoid kernel oops with bad gadgets

If a gadget driver loaded to a Superspeed-capable
peripheral controller, using a Superspeed cable,
doesn't provide Superspeed descriptors, we will get
a NULL pointer dereference.

In order to avoid that situation, we will try to
find any valid descriptors we can. If no set of
descriptors is passed in, then we'll let that gadget
oops anyhow.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>