iwlwifi: mvm: allow keeping connections in d3_test

Keeping connection can be useful also when testing d3
using the debugfs file (d3_test).

Save the vif to keep connection on, and consider it
when iterating over the interfaces.

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