net80211 amdpu: Simplify a few loops that drain an mbufq

These loops already handled a NULL return from mbufq_dequeue when the
queue was empty, so remove a redundant check of mbufq_len before
dequeueing.

Reviewed by:	bz
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D43336

sys/net80211/ieee80211_ht.c

@@ -517,7 +517,7 @@ ampdu_rx_purge_slot(struct ieee80211_rx_ampdu *rap, int i)
 	struct mbuf *m;
 
 	/* Walk the queue, removing frames as appropriate */
-	while (mbufq_len(&rap->rxa_mq[i]) != 0) {
+	for (;;) {
 		m = mbufq_dequeue(&rap->rxa_mq[i]);
 		if (m == NULL)
 			break;
@@ -812,7 +812,7 @@ ampdu_dispatch_slot(struct ieee80211_rx_ampdu *rap, struct ieee80211_node *ni,
 	struct mbuf *m;
 	int n = 0;
 
-	while (mbufq_len(&rap->rxa_mq[i]) != 0) {
+	for (;;) {
 		m = mbufq_dequeue(&rap->rxa_mq[i]);
 		if (m == NULL)
 			break;