wifi: iwlwifi: mvm: support new flush_sta method

For iwlwifi this is simple to implement, and on newer hardware
it's an improvement since we have per-station queues.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230620125813.a1f8ec20b727.I48594b708b41aa55dc2b8c3d346b4412ad3a5ba3@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>