ravb: Decrease TxFIFO depth of Q3 and Q2 to one

Hardware has the CBS (Credit Based Shaper) which affects only Q3
and Q2. When updating the CBS settings, even if the driver does so
after waiting for Tx DMA finished, there is a possibility that frame
data still remains in TxFIFO.

To avoid this, decrease TxFIFO depth of Q3 and Q2 to one.

This patch has been exercised this using netperf TCP_MAERTS, TCP_STREAM
and UDP_STREAM tests run on an Ebisu board. No performance change was
detected, outside of noise in the tests, both in terms of throughput and
CPU utilisation.

Fixes: c156633f ("Renesas Ethernet AVB driver proper")
Signed-off-by: NMasaru Nagai <masaru.nagai.vx@renesas.com>
Signed-off-by: NKazuya Mizuguchi <kazuya.mizuguchi.ks@renesas.com>
[simon: updated changelog]
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/renesas/ravb_main.c

@@ -458,7 +458,7 @@ static int ravb_dmac_init(struct net_device *ndev)
 		   RCR_EFFS | RCR_ENCF | RCR_ETS0 | RCR_ESF | 0x18000000, RCR);
 
 	/* Set FIFO size */
-	ravb_write(ndev, TGC_TQP_AVBMODE1 | 0x00222200, TGC);
+	ravb_write(ndev, TGC_TQP_AVBMODE1 | 0x00112200, TGC);
 
 	/* Timestamp enable */
 	ravb_write(ndev, TCCR_TFEN, TCCR);