Bluetooth: Classic: L2CAP: Fix issue that pending chan cannot be sent

When the returned buffer is a `NULL` of the pull function, it means
there is not any data needs to be sent. However maybe there is any
frame pending on other L2CAP channel needs to be sent over the same
ACL connection.

Re-trigger the TX processor. It will call the pull function again
and the pending buffer will be pulled from following L2CAP.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>

Author: lylezhu2012

subsys/bluetooth/host/classic/l2cap_br.c

@@ -1600,7 +1600,23 @@ struct net_buf *l2cap_br_data_pull(struct bt_conn *conn, size_t amount, size_t *
 
 #if defined(CONFIG_BT_L2CAP_RET_FC)
 	if (br_chan->tx.mode != BT_L2CAP_BR_LINK_MODE_BASIC) {
-		return l2cap_br_ret_fc_data_pull(conn, amount, length);
+		struct net_buf *buf;
+		const sys_snode_t *next_pdu_ready;
+
+		buf = l2cap_br_ret_fc_data_pull(conn, amount, length);
+
+		/* When the returned buffer is a `NULL`, it means there is not any data needs to
+		 * be sent. However maybe there is any frame pending on other L2CAP channel needs
+		 * to be sent over the same ACL connection. Re-trigger the TX processor. It will
+		 * call the function `l2cap_br_data_pull()` again and the pending buffer will be
+		 * pulled from following L2CAP.
+		 */
+		next_pdu_ready = sys_slist_peek_head(&conn->l2cap_data_ready);
+		if ((buf == NULL) && (next_pdu_ready != NULL) && (next_pdu_ready != pdu_ready)) {
+			bt_tx_irq_raise();
+		}
+
+		return buf;
 	}
 #endif /* CONFIG_BT_L2CAP_RET_FC */