nl80211: Try mode changes multiple times to avoid some races

cfg80211/mac80211 seems to be unwilling to change interface mode in
some cases. Make these less likely to cause problems by trying the
changes up to 10 times with 100 msec intervals.

src/drivers/driver_nl80211.c

@@ -4069,6 +4069,7 @@ static int wpa_driver_nl80211_set_mode(void *priv, int mode)
 	struct wpa_driver_nl80211_data *drv = bss->drv;
 	int ret = -1;
 	int nlmode;
+	int i;
 
 	switch (mode) {
 	case 0:
@@ -4101,11 +4102,23 @@ static int wpa_driver_nl80211_set_mode(void *priv, int mode)
 	 * take the device down, try to set the mode again, and bring the
 	 * device back up.
 	 */
-	if (linux_set_iface_flags(drv->ioctl_sock, bss->ifname, 0) == 0) {
-		/* Try to set the mode again while the interface is down */
-		ret = nl80211_set_mode(drv, drv->ifindex, nlmode);
-		if (linux_set_iface_flags(drv->ioctl_sock, bss->ifname, 1))
-			ret = -1;
+	wpa_printf(MSG_DEBUG, "nl80211: Try mode change after setting "
+		   "interface down");
+	for (i = 0; i < 10; i++) {
+		if (linux_set_iface_flags(drv->ioctl_sock, bss->ifname, 0) ==
+		    0) {
+			/* Try to set the mode again while the interface is
+			 * down */
+			ret = nl80211_set_mode(drv, drv->ifindex, nlmode);
+			if (linux_set_iface_flags(drv->ioctl_sock, bss->ifname,
+						  1))
+				ret = -1;
+			if (!ret)
+				break;
+		} else
+			wpa_printf(MSG_DEBUG, "nl80211: Failed to set "
+				   "interface down");
+		os_sleep(0, 100000);
 	}
 
 	if (!ret) {