USB: xhci: Fail gracefully if there's no SS ep companion descriptor.

This is a work around for a bug in the SuperSpeed Endpoint Companion Descriptor
parsing code.  It fails in some corner cases, which means ep->ss_ep_comp may be
NULL.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>