iwiwifi: mvm: Fix FW scan concurrency support assumptions

- The FW supports up to 4 concurrent scans, so adjust the definitions
  accordingly.
- Only a single periodic scan is supported, so enforce it in the code.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>