net: ethernet: ti: cpsw: fix ale port numbers

TI OMAP/Sitara SoCs have fixed number of ALE ports 3, which includes Host
port also.

Hence, use fixed value instead of value calcualted from DT, which can be
set by user and might not reflect actual HW configuration.

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