net: ethernet: ti: cpsw: use cpsw as drv data

No need to set ndev for drvdata when mainly cpsw reference is needed,
so correct this legacy decision.

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>