Do not try to recreate wlan(4) interface if it already exists.

This should fix error messages caused by devd(8) during startup:

Starting Network: lo0 wlan0.
...
Starting devd.
ifconfig: SIOCS80211: Device busy
wpa_supplicant already running?  (pid=323).

MFC after:	2 weeks

etc/pccard_ether

@@ -120,6 +120,9 @@ pccard_ether_restart()
 pccard_ether_startchildren()
 {
 	for child in `get_if_var $ifn wlans_IF`; do
+		if ifexists $child; then
+			continue
+		fi
 		/etc/rc.d/netif quietstart $child
 	done
 }