mmc: rtsx: check sg_count before long data xfer

Check sg_count before sending long data xfer.
Because dma_map_sg() return int, and sg_count may be negative,
so using int instead of unsigned.

Signed-off-by: Micky Ching <micky_ching@realsil.com.cn>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>