Move all the scan code that was in mac80211.c to scan.c where it
belongs, leaving only the parts that are specific to mac80211 ops.

Change some function definitions slightly to improve consistency.
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>