mac80211: Add DS Parameter Set into Probe Request on 2.4 GHz

IEEE Std 802.11k-2008 added DS Parameter Set information element into
Probe Request frames as an optional information on 2.4 GHz band (and
mandatory, if radio measurements are enabled). This allows APs to
filter out Probe Request frames that may be received from neighboring
overlapping channels and by doing so, reduce the number of unnecessary
frames in the air. Make mac80211 add this IE into Probe Request frames
whenever the channel is known (i.e., whenever hwscan is not used).

Signed-off-by: Jouni Malinen <j@w1.fi>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>