xlnx-bbram: hw/nvram: Fix uninitialized Error *

This adds required initialization of Error * variable.

Signed-off-by: Tong Ho <tong.ho@xilinx.com>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>