ptp: free 'vclock_index' in ptp_clock_release()

'vclock_index' is accessed from sysfs, it shouled be freed
in release function, so move it from ptp_clock_unregister()
to ptp_clock_release().

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>