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: NParav Pandit <parav@mellanox.com>
Signed-off-by: NLeon Romanovsky <leonro@mellanox.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>