nl80211: Introduce scan flags to emphasize requested scan behavior

This commit defines new scan flags (LOW_SPAN, LOW_POWER, HIGH_LATENCY)
to emphasize the requested scan behavior for the driver. These flags
are optional and are mutually exclusive. The implementation of the
respective functionality can be driver/hardware specific.

These flags can be used to control the compromise between how long
a scan takes, how much power it uses, and high accurate/complete
the scan is in finding the BSSs.

Signed-off-by: Sunil Dutt <usdutt@qti.qualcomm.com>
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>