cfg80211: fix wext setting SSID

Pavel reported that you can't set the SSID from "foo" to
"bar". I tried reproducing, but used different values,
with different lengths, and thus never saw the obvious
problem.

Reported-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>