lan743x: trim all 4 bytes of the FCS; not just 2

Trim all 4 bytes of the received FCS; not just 2 of them. Leaving 2
bytes of the FCS on the frame breaks DSA tailing tag drivers.

Fixes: a8db76d4 ("lan743x: boost performance on cpu archs w/o dma cache snooping")
Signed-off-by: George McCollister <george.mccollister@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/microchip/lan743x_main.c

@@ -2040,7 +2040,7 @@ lan743x_rx_trim_skb(struct sk_buff *skb, int frame_length)
 		dev_kfree_skb_irq(skb);
 		return NULL;
 	}
-	frame_length = max_t(int, 0, frame_length - RX_HEAD_PADDING - 2);
+	frame_length = max_t(int, 0, frame_length - RX_HEAD_PADDING - 4);
 	if (skb->len > frame_length) {
 		skb->tail -= skb->len - frame_length;
 		skb->len = frame_length;