net: ethernet: ti: am65-cpsw: fix rmii 100Mbit link mode

In RMII link mode it's required to set bit 15 IFCTL_A in MAC_SL MAC_CONTROL
register to enable support for 100Mbit link speed.

Fixes: 93a76530 ("net: ethernet: ti: introduce am65x/j721e gigabit eth subsystem driver")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>