RDMA/mlx5: Refactor affinity related code

Move affinity related code in modify qp to function.  It's a preparation
for next patch the extend the affinity calculation to consider the xmit
slave.

Link: https://lore.kernel.org/r/20200430192146.12863-16-maorg@mellanox.com
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>