iwlwifi: mvm: use only a single GTK in D3

Unfortunately, the firmware only supports replay counters for
a single GTK in D3, so that we should only upload the last
key and use its replay counters. Since mac80211 key iteration
will walk through the keys in order of their addition, simply
use the same HW key index (1) for all GTKs, thus overwriting
previous ones with newer ones. The replay counters for it are
already used.

Reviewed-by: Yaron Vaknin <Yaron.Vaknin@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>