cfg80211: increase scan result expire time

Using background scanning in mac80211 the time a scan needs to
finish can exceed 10 seconds. Hence, increase the scan results
expire time to 15 seconds which should be sufficient.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>