net: ethernet: ti: cpsw: use var instead of func for usage count

The usage count function is based on ndev_running flag that is
updated before calling ndo_open/close, but if ndo is called in
another place, as with suspend/resume, the counter is not changed,
that breaks sus/resume. For common resource no difference which
device is using it, does matter only device count. So, replace
usage count function on var and inc and dec it in ndo_open/close.

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