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)
Approved by: re (cperciva)
Differential Revision: https://reviews.freebsd.org/D47491

(cherry picked from commit 5399052c63a7a3d2f54615d31bfd092ab887a600)
(cherry picked from commit 7d0c12e1f907ddf9be6668449a966703c0f781d1)