net: wangxun: fix kernel panic due to null pointer

When the device uses a custom subsystem vendor ID, the function
wx_sw_init() returns before the memory of 'wx->mac_table' is allocated.
The null pointer will causes the kernel panic.

Fixes: 79625f45ca73 ("net: wangxun: Move MAC address handling to libwx")
Signed-off-by: Jiawen Wu <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/wangxun/libwx/wx_hw.c

···
 		wx->subsystem_device_id = pdev->subsystem_device;
 	} else {
 		err = wx_flash_read_dword(wx, 0xfffdc, &ssid);
-		if (!err)
-			wx->subsystem_device_id = swab16((u16)ssid);
+		if (err < 0) {
+			wx_err(wx, "read of internal subsystem device id failed\n");
+			return err;
+		}
 
-		return err;
+		wx->subsystem_device_id = swab16((u16)ssid);
 	}
 
 	wx->mac_table = kcalloc(wx->mac.num_rar_entries,

drivers/net/ethernet/wangxun/ngbe/ngbe_main.c

···
 
 	/* PCI config space info */
 	err = wx_sw_init(wx);
-	if (err < 0) {
-		wx_err(wx, "read of internal subsystem device id failed\n");
+	if (err < 0)
 		return err;
-	}
 
 	/* mac type, phy type , oem type */
 	ngbe_init_type_code(wx);

drivers/net/ethernet/wangxun/txgbe/txgbe_main.c

···
 
 	/* PCI config space info */
 	err = wx_sw_init(wx);
-	if (err < 0) {
-		wx_err(wx, "read of internal subsystem device id failed\n");
+	if (err < 0)
 		return err;
-	}
 
 	txgbe_init_type_code(wx);