phylink: avoid attaching more than one PHY

Attaching more than one PHY to phylink is bad news, as we store a
pointer to the PHY in a single location. Error out if more than one
PHY is attempted to be attached.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>