phy: sun4i-usb: fix dereference of pointer phy0 before it is null checked

Currently pointer phy0 is being dereferenced via the assignment of
phy on the call to phy_get_drvdata before phy0 is null checked, this
can lead to a null pointer dereference. Fix this by performing the
null check on phy0 before the call to phy_get_drvdata. Also replace
the phy0 == NULL check with the more usual !phy0 idiom.

Addresses-Coverity: ("Dereference before null check")
Fixes: e6f32efb ("phy: sun4i-usb: Make sure to disable PHY0 passby for peripheral mode")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20200625124428.83564-1-colin.king@canonical.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/phy/allwinner/phy-sun4i-usb.c

@@ -545,13 +545,14 @@ static void sun4i_usb_phy0_id_vbus_det_scan(struct work_struct *work)
 	struct sun4i_usb_phy_data *data =
 		container_of(work, struct sun4i_usb_phy_data, detect.work);
 	struct phy *phy0 = data->phys[0].phy;
-	struct sun4i_usb_phy *phy = phy_get_drvdata(phy0);
+	struct sun4i_usb_phy *phy;
 	bool force_session_end, id_notify = false, vbus_notify = false;
 	int id_det, vbus_det;
 
-	if (phy0 == NULL)
+	if (!phy0)
 		return;
 
+	phy = phy_get_drvdata(phy0);
 	id_det = sun4i_usb_phy0_get_id_det(data);
 	vbus_det = sun4i_usb_phy0_get_vbus_det(data);