iwlwifi: mvm: fix deduplication start logic

iwlwifi: mvm: fix deduplication start logic

If the first frame on a given TID is received with seqno 0 and needed
to be retransmitted, we erroneously drop it because the deduplication
data is initialized to zero, and then comparing

        if (unlikely(ieee80211_has_retry(hdr->frame_control) &&
                     dup_data->last_seq[tid] == hdr->seq_ctrl &&
                     dup_data->last_sub_frame[tid] >= sub_frame_idx))
                return true;

will return in iwl_mvm_is_dup() since last_sub_frame is also set to
zero, and sub_frame_idx is usually zero since this only covers the
relatively rare case of A-MSDU.

Fix this by initializing the last_seq array to 0xffff, which is an
impossible value for hdr->seq_ctrl to have here because the lower
four bits are the fragment number, and fragments aren't handled in
this code but go to mac80211 instead.

Fixes: a571f5f635ef ("iwlwifi: mvm: add duplicate packet detection per rx queue")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>