VMStateDescription.fields should be end with VMSTATE_END_OF_LIST().
However, microbit_i2c_vmstate doesn't follow it.  Let's change it.

Fixes: 9d68bf56 ("arm: Stub out NRF51 TWI magnetometer/accelerometer detection")
Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Peng Liang <liangpeng10@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20201019093401.2993833-1-liangpeng10@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>