r8169: check for PCI read error in probe

Check whether first PCI read returns 0xffffffff. Currently, if this is
the case, the user sees the following misleading message:
unknown chip XID fcf, contact r8169 maintainers (see MAINTAINERS file)

Signed-off-by: Heiner Kallweit <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

drivers/net/ethernet/realtek/r8169_main.c

···
 	int jumbo_max, region, rc;
 	enum mac_version chipset;
 	struct net_device *dev;
+	u32 txconfig;
 	u16 xid;
 
 	dev = devm_alloc_etherdev(&pdev->dev, sizeof (*tp));
···
 
 	tp->mmio_addr = pcim_iomap_table(pdev)[region];
 
-	xid = (RTL_R32(tp, TxConfig) >> 20) & 0xfcf;
+	txconfig = RTL_R32(tp, TxConfig);
+	if (txconfig == ~0U) {
+		dev_err(&pdev->dev, "PCI read failed\n");
+		return -EIO;
+	}
+
+	xid = (txconfig >> 20) & 0xfcf;
 
 	/* Identify chip attached to board */
 	chipset = rtl8169_get_mac_version(xid, tp->supports_gmii);