[PATCH] sata: fix sata_sx4 dma_prep to not use sg->length

 sata_sx4 directly references sg->length to calculate total_len in
pdc20621_dma_prep().  This is incorrect as dma_map_sg() could have
merged multiple sg's into one and, in such case, sg->length doesn't
reflect true size of the entry.  This patch makes it use
sg_dma_len(sg).

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>