mac80211: Remove WARN_ON_ONCE in ieee80211_recalc_smps

The recalc_smps work can run after the station disassociates.
At this stage we already released the channel, but the work
will be cancelled only when the interface stops.
In this scenario we can hit the warning in ieee80211_recalc_smps, so
just remove it.

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>