net: stmmac: xgmac: Remove uneeded computation for RFA/RFD

RFA and RFD should not be dependent on FIFO size. In fact, the more FIFO
space we have, the later we can activate Flow Control. Let's use
hard-coded values for RFA and RFD for all FIFO sizes with the exception
of 4k, which is a special case.
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/dwxgmac2_dma.c

@@ -183,19 +183,9 @@ static void dwxgmac2_dma_rx_mode(void __iomem *ioaddr, int mode,
 			rfa = 0x01; /* Full-1.5K */
 			break;
 
-		case 8192:
-			rfd = 0x06; /* Full-4K */
-			rfa = 0x0a; /* Full-6K */
-			break;
-
-		case 16384:
-			rfd = 0x06; /* Full-4K */
-			rfa = 0x12; /* Full-10K */
-			break;
-
 		default:
-			rfd = 0x06; /* Full-4K */
-			rfa = 0x1e; /* Full-16K */
+			rfd = 0x07; /* Full-4.5K */
+			rfa = 0x04; /* Full-3K */
 			break;
 		}