mac80211: Add support for aborting an ongoing scan

This commit adds implementation for abort scan in mac80211.

Reviewed-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Vidyullatha Kanchanapally <vkanchan@qti.qualcomm.com>
Signed-off-by: Sunil Dutt <usdutt@qti.qualcomm.com>
[adjust to wdev change in previous patch and clean up code a bit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>