net/cpsw: optimize the for_each_slave_macro()

text    data     bss     dec     hex filename
15530      92       4   15626    3d0a cpsw.o.before
15478      92       4   15574    3cd6 cpsw.o.after

52 bytes smaller, 13 for each invocation.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>