mdio_bus: Fix PTR_ERR applied after initialization to constant

Fix coccinelle warning:

./drivers/net/phy/mdio_bus.c:67:5-12: ERROR: PTR_ERR applied after initialization to constant on line 62
./drivers/net/phy/mdio_bus.c:68:5-12: ERROR: PTR_ERR applied after initialization to constant on line 62

Fix this by using IS_ERR before PTR_ERR

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 71dd6c0d ("net: phy: add support for reset-controller")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>