fix commutator in validate

kwant/physics/symmetry.py

@@ -166,9 +166,8 @@ class DiscreteSymmetry:
         for symm, conj, sign, name in zip(self[1:], _conj, _signs, _names):
             if symm is None:
                 continue
-            commutator = symm.T.conj().dot(cond_conj(symm.T.dot(matrix.T),
-                                                     conj).T)
-            commutator = commutator - sign * matrix
+            commutator = symm.T.conj().dot((symm.T.dot(matrix.T)).T)
+            commutator = commutator - sign * cond_conj(matrix, conj)
             if np.linalg.norm(commutator.data) > 1e-8:
                 return name