rocker: move ageing_time from struct rocker to struct ofdpa

This is OF-DPA specific, used only there, similar to
ofdpa_port->ageing_time. So move it to OF-DPA code.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>