iwlagn: use 6 Mbps rate for no-CCK scans

When userspace requested that a scan not be
done with CCK rates, use 6 Mbps. This is used
for example for P2P scanning.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>