mmc: omap_hsmmc: use original sg_len for dma_unmap_sg

Don't use the returned sg_len from dma_map_sg() as inparameter
to dma_unmap_sg(). Use the original sg_len for both dma_map_sg
and dma_unmap_sg according to the documentation in DMA-API.txt.

Signed-off-by: Per Forlin <per.forlin@linaro.org>
Reviewed-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>