The paper contained two mistakes: a minus sign is missed in Eq.(B22) and the plus sign should be replaced by a minus sign in Eq.(B23) of the AppendixB. The correct formulas are given below: (Formula Presented). These small corrections affect the numerical results of the paper for the nonsymmetrical case presented in Figs.4 and 5. Recalculated figures (Fig.1 and Fig. 2, respectively) and revised discussion are presented below.We thank S. Baturin for his interest in the paper and for fruitful discussions, which have helped us to discover the aforementioned mistakes.