net: ethernet: ti: cpsw: don't disable interrupts in ndo_open

No need to disable interrupts if no open devices,
they are disabled anyway.

Even no need to disable interrupts if some ndev is opened, In this
case shared resources are not touched, only parameters of ndev shell,
so no reason to disable them also. Removed lines have proved it.

So, no need in redundant check and interrupt disable.

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