iwlwifi: fix locking in delayed GTK setting

This code clearly never could have worked, since it locks
while already locked. Add an unlocked __iwl_mvm_mac_set_key()
variant that doesn't do locking to fix that.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>