r8152: Set memory to all 0xFFs on failed reg reads

get_registers() blindly copies the memory written to by the
usb_control_msg() call even if the underlying urb failed.

This could lead to junk register values being read by the driver, since
some indirect callers of get_registers() ignore the return values. One
example is:
  ocp_read_dword() ignores the return value of generic_ocp_read(), which
  calls get_registers().

So, emulate PCI "Master Abort" behavior by setting the buffer to all
0xFFs when usb_control_msg() fails.

This patch is copied from the r8152 driver (v2.12.0) published by
Realtek (www.realtek.com).

Signed-off-by: Prashant Malani <pmalani@chromium.org>
Acked-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/r8152.c

@@ -799,8 +799,11 @@ int get_registers(struct r8152 *tp, u16 value, u16 index, u16 size, void *data)
 	ret = usb_control_msg(tp->udev, usb_rcvctrlpipe(tp->udev, 0),
 			      RTL8152_REQ_GET_REGS, RTL8152_REQT_READ,
 			      value, index, tmp, size, 500);
+	if (ret < 0)
+		memset(data, 0xff, size);
+	else
+		memcpy(data, tmp, size);
 
-	memcpy(data, tmp, size);
 	kfree(tmp);
 
 	return ret;