net: fec_ptp: add clock rate zero check

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: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>