Bluetooth: host: Shrink size of struct bt_conn_tx

The `node` and `work` members are never used simultaneously.
Additionally k_work already has built-in support for being in a linked
list, however a union makes this change a bit cleaner.

Signed-off-by: Johan Hedberg <[email protected]>

Author: Johan Hedberg

subsys/bluetooth/host/conn_internal.h

@@ -85,9 +85,11 @@ struct bt_conn_tx_data {
 };
 
 struct bt_conn_tx {
-	sys_snode_t node;
+	union {
+		sys_snode_t node;
+		struct k_work work;
+	};
 	struct bt_conn *conn;
-	struct k_work work;
 	struct bt_conn_tx_data data;
 };