net: phy: Fix not to call phy_resume() if PHY is not attached

This patch fixes an issue that mdio_bus_phy_resume() doesn't call
phy_resume() if the PHY is not attached.

Fixes: 803dd9c7 ("net: phy: avoid suspending twice a PHY")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>