iwlwifi: mvm: rs: add ops for the new rate scaling in the FW

This patch introduces a new instance of rate_control_ops for
the new API (adding only empty stubs here and the subsequent
patches in the series will fill in the implementation).
The decision which API to use is done during the register
step according to FW TLV.

Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>