fix error command flag SD_FLAG_CMD_FILTER

All the macros are mutually exclusive in the SD_FLAG_CMD_XXX group,
so the new added flag cannot be set as 0x11 because
SD_FLAG_CMD_WRITE is already 0x01.

Secondly, the new added flag is renamed to SD_FLAG_CMD_PIGGYBACK
which stands for returning something back while sending something
to sheep.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>