net: enetc: fix array underflow in error handling code

This loop will try to unmap enetc_unmap_tx_buff[-1] and crash.

Fixes: 9d2b68cc ("net: enetc: add support for XDP_REDIRECT")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/YHBHfCY/yv3EnM9z@mwandaSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/freescale/enetc/enetc.c

@@ -895,7 +895,7 @@ static int enetc_xdp_frame_to_xdp_tx_swbd(struct enetc_bdr *tx_ring,
 		dma = dma_map_single(tx_ring->dev, data, len, DMA_TO_DEVICE);
 		if (unlikely(dma_mapping_error(tx_ring->dev, dma))) {
 			/* Undo the DMA mapping for all fragments */
-			while (n-- >= 0)
+			while (--n >= 0)
 				enetc_unmap_tx_buff(tx_ring, &xdp_tx_arr[n]);
 
 			netdev_err(tx_ring->ndev, "DMA map error\n");