USB: composite.h: mark private struct members as private:

Mark internal struct members as /* private: */ so that kernel-doc
won't produce warnings about missing descriptions for them.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>