iwlwifi: mvm: Support CSA countdown offloading

Add support CSA countdown offloading. When CSA starts, the driver
specifies the offsets to the eCSA and CSA IEs in the beacon template
command and the fw performs the countdown.
The fw notifies the driver when the channel switch flow
should be performed.
Beacon sent notifications are not used anymore.

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>