mac80211: Drop DS Channel PARAM in directed probe

Do not send DS Channel parameter for directed probe requests
in order to maximize the chance that we get a response.  Some
badly-behaved APs don't respond when this parameter is included.

Signed-off-by: Paul Stewart <pstew@chromium.org>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>