net: ravb: avoid dereferencing an invalid PTP clock

In the Linux kernel, the following vulnerability has been resolved: net: ravb: avoid dereferencing an invalid PTP clock The PTP clock is unavailable before the first open, so querying its index can dereference a NULL pointer. Registration failures can also leave an error pointer in priv->ptp.clock. Cache the PHC index separately and report -1 while no clock is registered. Normalize registration errors to NULL and preserve the static timestamping capabilities.