test: cmd: mbr: Fix Smatch static checker warning

This patch fixes Smatch static checker warning:
        test/cmd/mbr.c:243 mbr_test_run()
        warn: sizeof(NUMBER)?

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Alexander Gendin <agendin@matrox.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>