r8169: fix mdio_read and update mdio_write according to hw specs

Realtek confirmed that a 20us delay is needed after mdio_read and
mdio_write operations. Reduce the delay in mdio_write, and add it
to mdio_read too. Also add a comment that the 20us is from hw specs.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Acked-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/r8169.c

@@ -560,10 +560,10 @@ static void mdio_write(void __iomem *ioaddr, int reg_addr, int value)
 		udelay(25);
 	}
 	/*
-	 * Some configurations require a small delay even after the write
-	 * completed indication or the next write might fail.
+	 * According to hardware specs a 20us delay is required after write
+	 * complete indication, but before sending next command.
 	 */
-	udelay(25);
+	udelay(20);
 }
 
 static int mdio_read(void __iomem *ioaddr, int reg_addr)
@@ -583,6 +583,12 @@ static int mdio_read(void __iomem *ioaddr, int reg_addr)
 		}
 		udelay(25);
 	}
+	/*
+	 * According to hardware specs a 20us delay is required after read
+	 * complete indication, but before sending next command.
+	 */
+	udelay(20);
+
 	return value;
 }