{nl,cfg}80211: add a flags word to scan requests

Add a flags word to direct and scheduled scan requests; it will
be used for control of optional behaviours such as flushing the
bss cache prior to doing a scan.

Signed-off-by: Sam Leffler <sleffler@chromium.org>
Tested-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>