urtwn: heed rtsthreshold

[Stefan Sperling]

Same rts threshold fix as for rtwn(4).

Index: if_urtwn.c
===================================================================
RCS file: /cvs/src/sys/dev/usb/if_urtwn.c,v
retrieving revision 1.65
diff -u -p -r1.65 if_urtwn.c
--- if_urtwn.c  17 Jun 2016 10:53:55 -0000      1.65
+++ if_urtwn.c  20 Jul 2016 19:47:09 -0000
@@ -1299,7 +1299,10 @@ urtwn_tx(void *cookie, struct mbuf *m, s
                            SM(R92C_TXDW1_RAID, raid) | R92C_TXDW1_AGGBK);
                }
 
-               if (ic->ic_flags & IEEE80211_F_USEPROT) {
+               if (m->m_pkthdr.len + IEEE80211_CRC_LEN > ic->ic_rtsthreshold) {
+                       txd->txdw4 |= htole32(R92C_TXDW4_RTSEN |
+                           R92C_TXDW4_HWRTSEN);
+               } else if (ic->ic_flags & IEEE80211_F_USEPROT) {
                        if (ic->ic_protmode == IEEE80211_PROT_CTSONLY) {
                                txd->txdw4 |= htole32(R92C_TXDW4_CTS2SELF |
                                    R92C_TXDW4_HWRTSEN);