netifd: call "wifi down" before "wifi up"

When the underlying /etc/config/wireless got changed since the last "wifi up"
has been performed, the uci vap ifname state vars become inconsistent on a
subsequent "wifi up" and multiple vaps get mapped to the same ifnames which
confuses the gui and other processes relying on them.

For now call an explicit "wifi down" prior to each "wifi up" which will clear
up the state accordingly.

SVN-Revision: 33076