Fix duplicate wpa_supplicant(8) / hostapd(8) startup with devd(8)

Do not invoke 'wlan_up' function from devd(8) on interface
creation event (an example to create such event:
'ifconfig wlan0 create wlandev rtwn0');
they're typically produced during 'service netif (re)start'
and result in duplicate interface initialization.

From the user side if WPA option is used, this result in messages like:

• /etc/rc.d/wpa_supplicant: WARNING: failed to start wpa_supplicant

or

• wpa_supplicant already running? (pid=xxxx).

(for HOSTAP interfaces this race may result in startup failure).

As a side effect, wpa_supplicant(8) / hostapd(8) will not be
invoked when new wlan(4) interface is created manually and
corresponding configuration for it is present in rc.conf(5).

This change does not affect device attach / removal events.

MFC after: 5 days