drivers: net: davinci_mdio: remove busy loop on wait user access

Polling 14 mdio devices on single mdio bus eats 30% of 1Ghz cpu time
due to busy loop in wait(). Add small delay to relax cpu.

Signed-off-by: Max Uvarov <muvarov@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/ti/davinci_mdio.c

@@ -198,8 +198,10 @@ static inline int wait_for_user_access(struct davinci_mdio_data *data)
 			return 0;
 
 		reg = __raw_readl(&regs->control);
-		if ((reg & CONTROL_IDLE) == 0)
+		if ((reg & CONTROL_IDLE) == 0) {
+			usleep_range(100, 200);
 			continue;
+		}
 
 		/*
 		 * An emac soft_reset may have clobbered the mdio controller's