drivers: phy: Set phy->dev to NULL when generic_phy_get_by_index() fails

phy->dev need to be set to NULL in case of generic_phy_get_by_index()
fails. Then phy->dev can be used to check if the phy is valid

Reported-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/phy/phy-uclass.c

@@ -45,6 +45,7 @@ int generic_phy_get_by_index(struct udevice *dev, int index,
 	debug("%s(dev=%p, index=%d, phy=%p)\n", __func__, dev, index, phy);
 
 	assert(phy);
+	phy->dev = NULL;
 	ret = dev_read_phandle_with_args(dev, "phys", "#phy-cells", 0, index,
 					 &args);
 	if (ret) {