RDMA/core: Refactor ib_register_device() function

ib_register_device() does several allocation and initialization
steps. Split it into smaller more readable functions for easy
review and maintenance.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>