IB/mlx5: Move loopback initialization to the corresponding stage

The loopback stage only initializes a lock, move it to be in
the CAPS initialization phase and get rid loopback step completely.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>