RDMA/core: Use simpler device_del() instead of device_unregister()

Instead of holding extra reference using get_device() that
device_unregister() releases, simplify it as below.

device_add() balances with device_del().  device_initialize() balances
with put_device(), always via ib_dealloc_device().

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>