If a time event is already scheduled when trying to schedule one for
channel switch, the code assumes the channel switch is already
scheduled and no further action is required.
However, it is possible that the scheduled time event is actually
for session protection (e.g. when the first beacon after association
contains the CSA IE). In this case the channel switch will not be
scheduled which will finally lead to disconnection.

Fix this by removing the old time event and schduling a new one for
the channel switch.
Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>