drm/i915/gvt: Fix intel_vgpu_gpa_to_mmio_offset kernel docs

drm/i915/gvt: Fix intel_vgpu_gpa_to_mmio_offset kernel docs

intel_vgpu_gpa_to_mmio_offset states that it returns
'Zero on success, negative error code if failed'
in the kernel docs.

This is false.  The function actually returns
'The MMIO offset of the given GPA'.
Correct the docs.

Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw.linux@gmail.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/20250916174317.76521-6-jonathan.cavitt@intel.com