mac80211: support hardware TX fragmentation offload

The lower driver is notified when the fragmentation threshold changes
and upon a reconfig of the interface.

If the driver supports hardware TX fragmentation, don't fragment
packets in the stack.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/net/mac80211/util.c b/net/mac80211/util.c
index 0b6fc92..e486286 100644
--- a/net/mac80211/util.c
+++ b/net/mac80211/util.c
@@ -1152,6 +1152,9 @@
 	}
 	mutex_unlock(&local->sta_mtx);
 
+	/* setup fragmentation threshold */
+	drv_set_frag_threshold(local, hw->wiphy->frag_threshold);
+
 	/* setup RTS threshold */
 	drv_set_rts_threshold(local, hw->wiphy->rts_threshold);