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>