The Sunnah is the way or deeds of Muhammad and validated by the consensus of companions of Muhammad (Sahaba) in Sunni Islam, and the way or deeds of Muhammad and the twelve Imams in Shi'a Islam, while Hadith is a collection of the narrations and approvals.
Hadiths are classified according their status, in relation to their texts (matn) and their chain of transmitters (isnad).
Sunnah, on the other hand, is established through the practical examples and not via these texts in Islamic law, but mostly through the hadith texts as far as prophetic biography, traits and examples are concerned. For example, prayers, both individual and congregatory, were taught by Muhammad to his followers by practical example and since then have been transmitted generation-to-generation through practical learning. On the other hand, many traits about Muhammad, such as his style, his habits and his dealings with others, is known primarily through hadith.
I copied all of the above from here because I am lazy. What it says is correct, though.
So even if we believe Mohammad said anything positive about FGM (which is doubtful) it would be a hadith, because it would be just a comment he said. He did not teach it to others through practice, since none of his wives or daughters suffered FGM, therefore it is not sunna.
[I grew up in a Muslim country, which is why I am familiar with all this. It's not my belief, though.]