mac80211_hwsim: add sw_scan sw_scan_complete

Simple pre-scan and scan complete callbacks, this at least shows
to me that mac80211 will issue two scans at the same time on the
same wiphy.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>