iwlwifi: mvm: update power after phy_ctxt is NULL when unassigning chanctx

iwl_mvm_power_update_mac() (more specifically
iwl_mvm_power_iterator()) relies on the phy_ctxt setting to decide
whether the vif is active or not.  When unassigning a chanctx, we
should therefore call iwl_mvm_power_update_mac() after setting
phy_chanctx to NULL.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>