net: ethernet: ti: am65-cpsw-nuss: fix ports mac sl initialization

The MAC SL has to be initialized for each port otherwise
am65_cpsw_nuss_slave_disable_unused() will crash for disabled ports.

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