usb: dwc3: Use generic_phy_valid() helper

The documentation for struct phy state that "The content of the
structure is managed solely by the PHY API and PHY drivers".

Change to use the generic_phy_valid() helper to check if phy is valid.
Also remove setting phy->dev to NULL now that generic_phy_get_by_name()
properly initialize phy->dev to NULL.

Fixes: 142d50fbce7c ("usb: dwc3: Add support for usb3-phy PHY configuration")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
diff --git a/drivers/usb/dwc3/dwc3-generic.c b/drivers/usb/dwc3/dwc3-generic.c
index 7f0af05..3997b9d 100644
--- a/drivers/usb/dwc3/dwc3-generic.c
+++ b/drivers/usb/dwc3/dwc3-generic.c
@@ -541,8 +541,6 @@
 	} else if (ret != -ENOENT && ret != -ENODATA) {
 		debug("could not get phy (err %d)\n", ret);
 		return ret;
-	} else {
-		phy.dev = NULL;
 	}
 
 	glue->regs = dev_read_addr_size_index(dev, 0, &glue->size);
@@ -555,7 +553,7 @@
 	if (ret)
 		return ret;
 
-	if (phy.dev) {
+	if (generic_phy_valid(&phy)) {
 		ret = generic_phy_power_on(&phy);
 		if (ret)
 			return ret;