[iwn] Don't originate NULL data frames in net80211

Don't originate null data frames in net80211; leave that up to the
iwn firmware.

This should fix a bunch of cases where we're seeing out of sequence
802.11n aggregation data packets because net80211 power management
stuff was triggering NULL data frames when we don't need them and
messing up the firmware / ring buffer sequence number space.

Differential Revision:	https://reviews.freebsd.org/D52299

Locally tested:

* 6205, STA mode

sys/dev/iwn/if_iwn.c

@@ -586,6 +586,8 @@ iwn_attach(device_t dev)
 
 	/* Driver / firmware assigned sequence numbers */
 	ic->ic_flags_ext |= IEEE80211_FEXT_SEQNO_OFFLOAD;
+	/* Don't originate null data frames in net80211 */
+	ic->ic_flags_ext |= IEEE80211_FEXT_NO_NULLDATA;
 
 	/* Read MAC address, channels, etc from EEPROM. */
 	if ((error = iwn_read_eeprom(sc, ic->ic_macaddr)) != 0) {