nl80211: add api to abort a scan request on tx

nl80211: add api to abort a scan request on tx

Add NL80211_ATTR_SCAN_FLAGS to nl80211's NL80211_CMD_TRIGGER_SCAN and
define a flag to indicate outbound traffic should abort a scan request.

Signed-off-by: sleffler@chromium.org
BUG=chromium-os:11485
TEST=manual:run iw event -f in the background; run ping -i .25 <host>; wait for wpa_supplicant to issue a bgscan; note that the scan is aborted due to the ping; issue iw dev wlan0 scan and note the scan is not aborted and the ping packets are noticeably delayed

Change-Id: I27050f0e7b0485e2bc52f4dbd7fec9125436e238
Reviewed-on: http://gerrit.chromium.org/gerrit/5744
Reviewed-by: Gary Morain <gmorain@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Ready: Sam Leffler <sleffler@chromium.org>
Tested-by: Sam Leffler <sleffler@chromium.org>