ff3: fix maxLen calculation

[anitgandhi]

maxLen is incorrectly being calculated by moving the multiplication by 2 into the floor, whereas the spec calls for the multiplication to happen outside (after) the flooring.

This causes the bug referenced later in Encrypt, where the length of the input, n, is not allowed to be equal to maxLen, whereas it is allowed in the spec.

fpe/ff3/ff3.go

Line 87 in e6bfa0d

maxLen := uint32(math.Floor((192 / math.Log2(float64(radix)))))