People have already said that suf- is a form of sub- (it can also be found as suc-, sug-, sum-, sup-, sur- and sus-).
What they left out is that it's opposite is super- or sur-. Yes, that does mean that one of the forms of sub- is exactly the same as one of the forms of its opposite! Languages that form organically over thousands of years aren't always logical.
A lot of cases with sub- and its variants have no logical opposite though, since e.g. subcutaneous means below the skin, but "above the skin" makes no sense, since we don't have any bit of us above the skin.
The example you give though, is an exception again. As suffix's antonym is prefix, because they don't have perfectly corresponding component parts. Again, English isn't always logical.