iwx: re-add sequence numbers for non-AMPDU, preserving previous behaviour

I received a report on freebsd-wireless@ that traffic stopped working.
It seems that although my AX210 is fine with this change, the AX201/22000
hardware/firmware seems to require it.

There's a bit to set in the TX command to force the FW to set the
sequence control.  We only set a few flags, and we don't set that.

So, add the sequence number allocation where encryption is performed.

Locally tested:

* AX210, STA mode
* AX201, STA mode

Differential Revision:	https://reviews.freebsd.org/D52652
Reviewed by:	kevans
Tested by:	kevans

sys/dev/iwx/if_iwx.c

@@ -5673,8 +5673,9 @@ iwx_tx(struct iwx_softc *sc, struct mbuf *m, struct ieee80211_node *ni)
 	if (rinfo == NULL)
 		return EINVAL;
 
-	/* Offloaded sequence number assignment */
+	/* Offloaded sequence number assignment; non-AMPDU case */
-	/* Note: Should be done in firmware on all supported devices */
+	if ((m->m_flags & M_AMPDU_MPDU) == 0)
+		ieee80211_output_seqno_assign(ni, -1, m);
 
 	/* Radiotap */
 	if (ieee80211_radiotap_active_vap(vap)) {