net: ethernet: ti: cpsw: simplify submit routine

As second net dev is created only in case of dual_emac mode, port
number can be figured out in simpler way. Also no need to pass
redundant ndev struct.

Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>