Make iwn(4) pass control frames in monitor mode

[Stefan Sperling]

This allows tcpdump to see all control frames with iwn(4).

Index: if_iwn.c
===================================================================
RCS file: /cvs/src/sys/dev/pci/if_iwn.c,v
retrieving revision 1.158
diff -u -p -r1.158 if_iwn.c
--- if_iwn.c    25 Jan 2016 11:27:11 -0000      1.158
+++ if_iwn.c    3 Feb 2016 16:01:58 -0000
@@ -2007,8 +2007,11 @@ iwn_rx_done(struct iwn_softc *sc, struct
                ifp->if_ierrors++;
                return;
        }
-       /* Discard frames that are too short. */
-       if (len < sizeof (*wh)) {
+       /* 
+        * Discard frames that are too short, unless in monitor mode where we
+        * receive control frames which are shorter than regular data frames.
+        */
+       if (len < sizeof (*wh) && ic->ic_opmode != IEEE80211_M_MONITOR) {
                DPRINTF(("frame too short: %d\n", len));
                ic->ic_stats.is_rx_tooshort++;
                ifp->if_ierrors++;