net: stmmac: make sure that ptp_rate is not 0 before configuring EST

In the Linux kernel, the following vulnerability has been resolved: net: stmmac: make sure that ptp_rate is not 0 before configuring EST If the ptp_rate recorded earlier in the driver happens to be 0, this bogus value will propagate up to EST configuration, where it will trigger a division by 0. Prevent this division by 0 by adding the corresponding check and error code.