iwlwifi: mvm: support new beacon template command

Support a new version of the beacon template command. This replaces v8
of the command, which was missing the rate code.  Also, export rate
decision logic to a separate function.

Signed-off-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>