drm/gpuvm: Fix kernel-doc warning for drm_gpuvm_map_req.map

drm/gpuvm: Fix kernel-doc warning for drm_gpuvm_map_req.map

The kernel-doc for struct drm_gpuvm_map_req.map was added as '@op_map'
instead of '@map', leading to this warning during htmldocs build:

WARNING: include/drm/drm_gpuvm.h:1083 struct member 'map' not described in 'drm_gpuvm_map_req'

Fixes: 000a45dce7ad ("drm/gpuvm: Pass map arguments through a struct")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/all/20250821133539.03aa298e@canb.auug.org.au/
Signed-off-by: Ankan Biswas <spyjetfayed@gmail.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>