mac80211: fix IBSS join by initializing last_scan_completed

commit c7d37a66e345df2fdf1aa7b2c9a6d3d53846ca5b upstream.

Without this fix, freshly rebooted Linux creates a new IBSS
instead of joining an existing one. Only when jiffies counter
overflows after 5 minutes the IBSS can be successfully joined.

Signed-off-by: Krzysztof Hałasa <khalasa@piap.pl>
[edit commit message slightly]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>