wl12xx: check the vif's operstate after join

When resuming while connected (without wowlan), the interface
is already IF_OPER_UP, so we won't get the notifier callback,
and hence never complete the association (from wl12xx perspective)

This situation, among other potential problems, prevents the
station from entering psm.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>