RDMA/cma: Make sure that PSN is not over max allowed

This patch limits the initial value for PSN to 24 bits as
spec requires.

Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Mukesh Kacker <mukesh.kacker@oracle.com>
Signed-off-by: Daniel Jurgens <danielj@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>