ptp: Ensure info->enable callback is always set

In the Linux kernel, the following vulnerability has been resolved: ptp: Ensure info->enable callback is always set The ioctl and sysfs handlers unconditionally call the ->enable callback. Not all drivers implement that callback, leading to NULL dereferences. Example of affected drivers: ptp_s390.c, ptp_vclock.c and ptp_mock.c. Instead use a dummy callback if no better was specified by the driver.