The right to free speach protects us from punishment by the government for something we have said. There are limits. Some of Churchill's statements advocating violence push those limits. But he is not being arrested, so the first amendment doesn't apply.
Getting fired is a wholely different thing. Acedemic freedom would apply, if he were speaking as an academic about his field of expertise. He was not. Tenure is not imunity from moral turpitude. Let a tenured professor advocate the rape and dismemberment of coeds, and see how long he is employed.
Meh, we'll see. Professors have been writing on contreversial topics outside of their fields for years. Alfred Kinsey was an entomologist before he started his sex surveys. And Noam Chomsky is, by occupation, a lingusit, (hell of a linguist, actually) but he's best known for his social commentary.
Now, to be fair, Churchill isn't anywhere near as brilliant as those two examples, but he is a tenured professor. Historically, tenure would protect Churchill in this situation and I don't think that things will be any different this time.