net: ethernet: ti: fix possible object reference leak

The call to of_get_child_by_name returns a node pointer with refcount
incremented thus it must be explicitly decremented after the last
usage.

Detected by coccinelle with the following warnings:
./drivers/net/ethernet/ti/netcp_ethss.c:3661:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 3654, but without a corresponding object release within this function.
./drivers/net/ethernet/ti/netcp_ethss.c:3665:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 3654, but without a corresponding object release within this function.

Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Cc: Wingman Kwok <w-kwok2@ti.com>
Cc: Murali Karicheri <m-karicheri2@ti.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>