The MAC address set from Qemu wasn't being saved into the register space.

Reviewed-by: Hao Wu <wuhaotsh@google.com>
Signed-off-by: Patrick Venture <venture@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: moved variable declaration to top of function]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>