net: sdk_fman: avoid division by zero

commit  64960d896c1cb6efaea6bebe53e9c4a96c7e68ab from
https://github.com/nxp-imx/linux-imx

 lf-6.1.y

Check clkCnt before dividing.

Signed-off-by: Camelia Groza <camelia.groza@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>