ravb: Remove the macros NUM_TX_DESC_GEN[23]

For addressing 4 bytes alignment restriction on transmission
buffer for R-Car Gen2 we use 2 descriptors whereas it is a single
descriptor for other cases.
Replace the macros NUM_TX_DESC_GEN[23] with magic number and
add a comment to explain it.
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Suggested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/renesas/ravb.h

@@ -956,10 +956,6 @@ enum RAVB_QUEUE {
 
 #define RX_BUF_SZ	(2048 - ETH_FCS_LEN + sizeof(__sum16))
 
-/* TX descriptors per packet */
-#define NUM_TX_DESC_GEN2	2
-#define NUM_TX_DESC_GEN3	1
-
 struct ravb_tstamp_skb {
 	struct list_head list;
 	struct sk_buff *skb;

drivers/net/ethernet/renesas/ravb_main.c

@@ -2160,8 +2160,12 @@ static int ravb_probe(struct platform_device *pdev)
 	ndev->max_mtu = 2048 - (ETH_HLEN + VLAN_HLEN + ETH_FCS_LEN);
 	ndev->min_mtu = ETH_MIN_MTU;
 
-	priv->num_tx_desc = info->aligned_tx ?
-		NUM_TX_DESC_GEN2 : NUM_TX_DESC_GEN3;
+	/* FIXME: R-Car Gen2 has 4byte alignment restriction for tx buffer
+	 * Use two descriptor to handle such situation. First descriptor to
+	 * handle aligned data buffer and second descriptor to handle the
+	 * overflow data because of alignment.
+	 */
+	priv->num_tx_desc = info->aligned_tx ? 2 : 1;
 
 	/* Set function */
 	ndev->netdev_ops = &ravb_netdev_ops;