dsa: lan9303: write reg only if necessary

As the regmap_write() is over a slow bus that will sleep, we can speed up
the boot-up time a bit by not bothering to clear a bit that is already
clear.
Signed-off-by: Jerry Ray <jerry.ray@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/lan9303-core.c

@@ -915,8 +915,11 @@ static int lan9303_setup(struct dsa_switch *ds)
 	if (ret)
 		return (ret);
 
+	/* Clear the TURBO Mode bit if it was set. */
+	if (reg & LAN9303_VIRT_SPECIAL_TURBO) {
 		reg &= ~LAN9303_VIRT_SPECIAL_TURBO;
 		regmap_write(chip->regmap, LAN9303_VIRT_SPECIAL_CTRL, reg);
+	}
 
 	ret = lan9303_setup_tagging(chip);
 	if (ret)