iwlwifi: mvm: reset ucode_loaded flag when mac80211 stop is called

We rely on the value of the mvm->ucode_loaded flag to decide whether
or not we can perform certain operations (e.g. access to some debugfs
entries), so we need to reset it when the mac80211 stop operation is
called and the hardware is shutdown.

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>