iwlwifi: mvm: add debugfs entry with the number of net-detect scans

Our testers need to know the number of scans performed while in
net-detect mode before the device wakes up.  The firmware already
passes this information to the driver, so we can save it and report it
in a debugfs entry.

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