drm/amdgpu: Fix an error code in kfd_mem_attach_dmabuf()

If amdgpu_gem_prime_export() fails, then this code accidentally
returns zero/success instead of a negative error code.

Fixes: 5ac3c3e4 ("drm/amdgpu: Add DMA mapping of GTT BOs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>