net: ethernet: ti: cpsw: fix crash in dual_emac mode with one ext port

commit 54af69ffaabcb70689612cef34265de72d4cf809 from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git



The CPSW driver will crash with NULL pointer exception in dual_emac mode
when there is only one ext. port physically connected and DT property
"slaves" set to <1>.

Fix crash, by adding missing check to prevent creating second netdev when
"slaves = <1>".

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>