asm/dma-mapping-common: Clarify output of dma_map_sg_attrs

asm/dma-mapping-common: Clarify output of dma_map_sg_attrs

Although dma_map_sg_attrs returns 0 on error and it cannot return a
value < 0, the function returns a signed integer.

Most of the time, this function is used with a scatterlist structure.
This structure uses an unsigned integer for the number of memory.

A dma developer that has not read in detail DMA-API.txt, can wrongly
return a value < 0 on error.

The comment will help the driver developer, and the WARN_ON the dma
developer.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>