net: ethernet: ti: cpsw: remove dual check from common res usage function

Common res usage is possible only in case an interface is
running. In case of not dual emac here can be only one interface,
so while ndo_open and switch mode, only one interface can be opened,
thus if open is called no any interface is running ... and no common
res are used. So remove check on dual emac, it will simplify
code/understanding and will match the name it's called.

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