At present there is a data_ready() callback for the SD data read
path. Let's add a receive_ready() for the SD data write path.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20210128063035.15674-5-bmeng.cn@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>