mdio_bus: suppress err message for reset gpio EPROBE_DEFER

The mdio_bus may have dependencies from GPIO controller and so got
deferred. Now it will print error message every time -EPROBE_DEFER is
returned which from:
__mdiobus_register()
 |-devm_gpiod_get_optional()
without actually identifying error code.

"mdio_bus 4a101000.mdio: mii_bus 4a101000.mdio couldn't get reset GPIO"

Hence, suppress error message for devm_gpiod_get_optional() returning
-EPROBE_DEFER case by using dev_err_probe().
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Link: https://lore.kernel.org/r/20201119203446.20857-1-grygorii.strashko@ti.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/mdio_bus.c

@@ -546,10 +546,11 @@ int __mdiobus_register(struct mii_bus *bus, struct module *owner)
 	/* de-assert bus level PHY GPIO reset */
 	gpiod = devm_gpiod_get_optional(&bus->dev, "reset", GPIOD_OUT_LOW);
 	if (IS_ERR(gpiod)) {
-		dev_err(&bus->dev, "mii_bus %s couldn't get reset GPIO\n",
+		err = dev_err_probe(&bus->dev, PTR_ERR(gpiod),
+				    "mii_bus %s couldn't get reset GPIO\n",
 				    bus->id);
 		device_del(&bus->dev);
-		return PTR_ERR(gpiod);
+		return err;
 	} else	if (gpiod) {
 		bus->reset_gpiod = gpiod;