Bluetooth: mgmt: Fix MGMT add advmon with RSSI command

The MGMT command: MGMT_OP_ADD_ADV_PATTERNS_MONITOR_RSSI uses variable
length argument. This causes host not able to register advmon with rssi.

This patch has been locally tested by adding monitor with rssi via
btmgmt on a kernel 6.1 machine.
Reviewed-by: Archie Pusaka <apusaka@chromium.org>
Fixes: b338d917 ("Bluetooth: Implement support for Mesh")
Signed-off-by: Howard Chung <howardchung@google.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

net/bluetooth/mgmt.c

@@ -9357,7 +9357,8 @@ static const struct hci_mgmt_handler mgmt_handlers[] = {
 	{ add_ext_adv_data,        MGMT_ADD_EXT_ADV_DATA_SIZE,
 						HCI_MGMT_VAR_LEN },
 	{ add_adv_patterns_monitor_rssi,
-				   MGMT_ADD_ADV_PATTERNS_MONITOR_RSSI_SIZE },
+				   MGMT_ADD_ADV_PATTERNS_MONITOR_RSSI_SIZE,
+						HCI_MGMT_VAR_LEN },
 	{ set_mesh,                MGMT_SET_MESH_RECEIVER_SIZE,
 						HCI_MGMT_VAR_LEN },
 	{ mesh_features,           MGMT_MESH_READ_FEATURES_SIZE },