iwlwifi: increase scan timeout

When the first interface is active, then scanning
on it or the second interface can take a little
longer than 7s (I observed around 8s.) Bump the
timeout to 15s to avoid aborting a scan that is
still running, just taking more time.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>