phy: Allow a NULL phy name for devm_phy_get()

For a single PHY, there's no reason to have a phy-names entry in DT.
The DT specific get functions allow for this already, but devm_phy_get()
WARNs in this case. Other subsystems also don't warn in their get
functions. Let's drop the WARN for DT case in devm_phy_get().

Cc: Kishon Vijay Abraham I <[e-mail]>
Cc: Vinod Koul <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

drivers/phy/phy-core.c

···
 	struct phy *phy;
 	struct device_link *link;
 
-	if (string == NULL) {
-		dev_WARN(dev, "missing string\n");
-		return ERR_PTR(-EINVAL);
-	}
-
 	if (dev->of_node) {
-		index = of_property_match_string(dev->of_node, "phy-names",
-			string);
+		if (string)
+			index = of_property_match_string(dev->of_node, "phy-names",
+				string);
+		else
+			index = 0;
 		phy = _of_phy_get(dev->of_node, index);
 	} else {
+		if (string == NULL) {
+			dev_WARN(dev, "missing string\n");
+			return ERR_PTR(-EINVAL);
+		}
 		phy = phy_find(dev, string);
 	}
 	if (IS_ERR(phy))