net: phy: fixed_phy: print gpio error only if gpio node is present

It is perfectly ok to not have an gpio attached to the fixed-link node. So
the driver should not throw an error message when the gpio is missing.

Fixes: 5468e82f ("net: phy: fixed-phy: Drop GPIO from fixed_phy_add()")
Signed-off-by: Hubert Feurstein <h.feurstein@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>