net: ethernet: ti: cpts: clean up event list if event pool is empty

When a CPTS user does not exit gracefully by disabling cpts
timestamping and leaving a joined multicast group, the system
continues to receive and timestamps the ptp packets which eventually
occupy all the event list entries.  When this happns, the added code
tries to remove some list entries which are expired.

Signed-off-by: WingMan Kwok <w-kwok2@ti.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>