iwlwifi: mvm: Don't request HW restart if already requested

In case mac80211 was requested to perform an HW restart, but the HW
restart has not started yet, there is no need to request another one.

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