p54: fix tx feedback status flag check

Michael reported that p54* never really entered power save mode, even tough it was enabled.

It turned out that upon a power save mode change the firmware will set a special flag onto the last outgoing frame tx status (which in this case is almost always the designated PSM nullfunc frame). This flag confused the driver; It erroneously reported transmission failures to the stack, which then generated the next nullfunc. and so on...

---

index 34447d1..1dc0dbf 100644
@@ -446,7 +446,7 @@ static void p54_rx_frame_sent(struct p54_common *priv, struct sk_buff *skb)
 	}
 
 	if (!(info->flags & IEEE80211_TX_CTL_NO_ACK) &&
-	     (!payload->status))
+	     !(payload->status & P54_TX_FAILED))
 		info->flags |= IEEE80211_TX_STAT_ACK;
 	if (payload->status & P54_TX_PSM_CANCELLED)
 		info->flags |= IEEE80211_TX_STAT_TX_FILTERED;