net: phy: don't touch suspended flag if there's no suspend/resume callback

So far we set phydev->suspended to true in phy_suspend() even if the
PHY driver doesn't implement the suspend callback. This applies
accordingly for the resume path. The current behavior doesn't cause
any issue I'd be aware of, but it's not logical and misleading,
especially considering the description of the flag:
"suspended: Set to true if this phy has been suspended successfully"

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>