net: ethernet: ti: cpsw: don't duplicate ndev_running

No need to create additional vars to identify if interface is running.
So simplify code by removing redundant var and checking usage counter
instead.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>