dma-mapping: Fix warning reported for missing prototype

lkp reported a warning about missing prototype for a recent patch.

The kernel-doc style comments are out of sync, move them to the right
function.

Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Christoph Hellwig <hch@lst.de>

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202504190615.g9fANxHw-lkp@intel.com/



Signed-off-by: Balbir Singh <balbirs@nvidia.com>
[mszyprow: reformatted subject]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20250422114034.3535515-1-balbirs@nvidia.com