mac80211: fix memory leak in ieee80211_send_probe_req

This patch fixes memory leak in ieee80211_send_probe_req, which
is introduced in 7c12ce8b:

	mac80211: use Probe Request template when sending a direct scan

The patch is against the latest wireless-test tree.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Reviewed-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>