bsdinstall: wlanconfig: fix interface UP on (re-)starting wpa_supplicant

Make sure an interface is back up before (re-)starting wpa_supplicant
in wlanconfig not relying on wpa to UP the interface (though we fixed
that).

Sponsored by: The FreeBSD Foundation
Reviewed by: emaste (in D47491)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D47491