IB/mlx5: Refactor mlx5_ib_qp to accommodate other QP types

Extract specific IB QP fields to mlx5_ib_qp_trans structure.
The mlx5_core QP object resides in mlx5_ib_qp_base, which all QP types
inherit from. When we need to find mlx5_ib_qp using mlx5_core QP
(event handling and co), we use a pointer that resides in
mlx5_ib_qp_base.

In addition, we delete all redundant fields that weren't used anywhere
in the code:
-doorbell_qpn
-sq_max_wqes_per_wr
-sq_spare_wqes

Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Reviewed-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>