dma-coherent: clarify dma_mmap_from_dev_coherent documentation

The use of "correctly mapped" here is misleading, since it can give the
wrong expectation in the case that the memory *should* have been mapped
from the per-device pool, but doing so failed for other reasons.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>