hw/sd: sd: Move the sd_block_{read, write} and macros ahead

These APIs and macros may be referenced by functions that are
currently before them. Move them ahead a little bit.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210216150225.27996-5-bmeng.cn@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>