mac80211: fix an off-by-one issue in A-MSDU max_subframe computation

Initialize 'n' to 2 in order to take into account also the first
packet in the estimation of max_subframe limit for a given A-MSDU
since frag_tail pointer is NULL when ieee80211_amsdu_aggregate
routine analyzes the second frame.

Fixes: 6e0456b54545 ("mac80211: add A-MSDU tx support")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -3174,7 +3174,7 @@ static bool ieee80211_amsdu_aggregate(struct ieee80211_sub_if_data *sdata,
 	void *data;
 	bool ret = false;
 	unsigned int orig_len;
-	int n = 1, nfrags, pad = 0;
+	int n = 2, nfrags, pad = 0;
 	u16 hdrlen;
 
 	if (!ieee80211_hw_check(&local->hw, TX_AMSDU))