iwlwifi: mvm: clean up scan capability checks

Introduce and use iwl_mvm_cdb_scan_api(), which checks the family.
Most of this will go away once the 22000 firmware supports adaptive
dwell, after which the V6 scan API won't be used, but the V3 scan
*config* API will still need to be distinguished.

In any case, this gets rid of the completely bogus has_new_tx_api()
checks.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>