mac80211: fix IBSS join by initializing last_scan_completed

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]
Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>