iwlwifi: mvm: Fix channel switch in case of count <= 1

The code did not consider the case that the channel switch counter
is <= 1, which would result with an inaccurate calculation of the
time event apply time.

As the specification states that in case of counter == 0 the switch
occurs at any time after the reception the frame, and for counter == 1
the switch would happens before the next TBTT, schedule the time
event immediately.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>