[W]here the defendant uses a trademark to describe the plaintiff's product, rather than its own, we hold that a commercial user is entitled to a nominative fair use defense provided he meets the following three requirements:
- the product or service in question must be one not readily identifiable without use of the trademark;
- only so much of the mark or marks may be used as is reasonably necessary to identify the product or service; and
- the user must do nothing that would, in conjunction with the mark, suggest sponsorship or endorsement by the trademark holder.