RDMA/mlx5: Do not generate the uabi specs unconditionally

For DM there is no reason not to add the spec for the START_OFFSET, if DM
is not supported then ib_dev.alloc_dm is already set to NULL which ensures
we do not call the method.

For IPSEC, the core code should be setting ib_dev.create_flow_action_esp
to NULL to disable it, not relying on wonky manipulation of the specs.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>