sparc/iommu: fix ->map_sg return value

Just decrementing the sz value will lead to an incorrect return value.
Instead of just introducing a local variable switch to the standard
for_each_sg helper and standard naming of the arguments.

Fixes: ce65d36f ("sparc: remove the sparc32_dma_ops indirection")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>