iwlwifi: mvm: Implement low-priority scan

Advertise driver's support for low priority scan.
Notice that this overwrites current setting by mac80211 which depends
only on hw scan support.
This scan priority  can be configured by user space application
and it affects scan continuity, low priority scan
will be more fragmented scan.

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