Add clock rate zero check to fix coverity issue of "divide by 0".

Fixes: commit 85bd1798 ("net: fec: fix spin_lock dead lock")
Signed-off-by: NFugang Duan <fugang.duan@nxp.com>
Signed-off-by: NJoakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>