vdpa/mlx5: Directly assign memory key

When creating a memory key, the key value should be assigned to the
passed pointer and not or'ed to.

No functional issue was observed due to this bug.

Fixes: 29064bfd ("vdpa/mlx5: Add support library for mlx5 VDPA implementation")
Signed-off-by: Eli Cohen <elic@nvidia.com>
Message-Id: <20230205072906.1108194-1-elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>