iwlwifi: mvm: add back support for low-priority scan

The low-priority scan feature can be useful, e.g. for OBSS
scans (if those are required by the AP); add back support
for it, restoring the maximum out time to the value it was
for low-priority scan before that was removed.

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