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.