net: ethernet: ti: cpsw: don't check slave num in runtime

No need to check const slave num in runtime for every packet,
and ndev for slaves w/o ndev is anyway NULL. So remove redundant
check and macro.

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