case AX25_T1:
-		if (opt < 1) {
+		if (opt < 1 || opt > 30) {

Where do these values come from ? If they are from some 'standard' then
really we should avoid restricting needlessly to it, particularly as
AX.25 isn't well defined and is used for all sorts of crazy stuff where
the usual range of settings isn't useful.

Restricting to the point it would overflow makes sense however.

Alan