net: fec_ptp: add clock rate zero check

commit cb3cefe3 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: Idf85b5ae13f810918edb00d92dbf54e5312f1eb8