net: lan966x: Fix use of pointer after being freed

The smatch found the following warning:

drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c:736 lan966x_fdma_reload()
warn: 'rx_dcbs' was already freed.

This issue can happen when changing the MTU on one of the ports and once
the RX buffers are allocated and then the TX buffer allocation fails.
In that case the RX buffers should not be restore. This fix this issue
such that the RX buffers will not be restored if the TX buffers failed
to be allocated.

Fixes: 2ea1cbac ("net: lan966x: Update FDMA to change MTU.")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Link: https://lore.kernel.org/r/20220511204059.2689199-1-horatiu.vultur@microchip.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c

@@ -729,11 +729,11 @@ static int lan966x_fdma_reload(struct lan966x *lan966x, int new_mtu)
 	return err;
 restore:
 	lan966x->rx.dma = rx_dma;
-	lan966x->tx.dma = tx_dma;
+	lan966x->rx.dcbs = rx_dcbs;
 	lan966x_fdma_rx_start(&lan966x->rx);
 
 restore_tx:
-	lan966x->rx.dcbs = rx_dcbs;
+	lan966x->tx.dma = tx_dma;
 	lan966x->tx.dcbs = tx_dcbs;
 	lan966x->tx.dcbs_buf = tx_dcbs_buf;