RDMA/core: Change system parameters callback from dumpit to doit

.dumpit() callback is used for returning same type of data in the loop,
e.g. loop over ports, resources, devices.

However system parameters are general and standalone for whole
subsystem. It means that getting system parameters should be doit
callback.

Fixes: cb7e0e13 ("RDMA/core: Add interface to read device namespace sharing mode")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>