drivers: net: ethernet: cpsw: change cpts_active_slave to active_slave

Change cpts_active_slave to active_slave so that the same DT property
can be used to ethtool and SIOCGMIIPHY.

CC: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>