drivers: dma-coherent: Fix typo in dma_mmap_from_coherent documentation

The function documentation incorrectly references dma_release_coherent.
Fix it. Don't mention a specific function name as dma_mmap_from_coherent
as multiple callers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>