wifi: iwlwifi: mvm: fix iwl_mvm_mac_flush_sta()

When I implemented iwl_mvm_mac_flush_sta() I completely botched it;
it basically always happens after the iwl_mvm_sta_pre_rcu_remove()
call, and that already clears mvm->fw_id_to_mac_id[] entries, so we
cannot rely on those at iwl_mvm_mac_flush_sta() time. This means it
never did anything.

Fix this by just going through the station IDs and now with the new
API for iwl_mvm_flush_sta(), call those.

Fixes: a6cc6ccb

 ("wifi: iwlwifi: mvm: support new flush_sta method")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20231011130030.0b5878e93118.I1093e60163052e7be64d2b01424097cd6a272979@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>