drivers: net: cpsw: fix suspend when all ethX devices are down

The cpsw_suspend() could trigger L3 error and CPSW will stop
functioning if System enters suspend when all ethX net-devices are
down - in this case CPSW could be already suspended by PM runtime, but
cpsw_suspend() will try to call soft_reset_slave() unconditionally
and access CPSW registers.

Hence, fix it by moving soft_reset_slave() from cpsw_suspend() to
cpsw_slave_stop(). This way slave ports will be reset when CPSW is
active and will be in proper state during Suspend.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>