Add an absent time event when pre_channel_switch is called and use the
time event started indication to set the disable_tx bit instead of
doing it in unassign_vif().  This is done so that the firmware queues
are stopped before the actual switch takes place to avoid losing
packets while the AP/GO is performing its actual switch.
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>