IB/mlx4: Refer to the device kobject instead of ports_parent

iov sysfs tree is created under ib device at
/sys/class/infiniband/mlx4_0/iov.
And,
ibdev->ports_parent->parent = &ibdev->dev.

Therefore, refer to device's kobject directly instead of
indirect access to it.

Additionally, iov entries are created under device kobject and deleted
before device is removed. There is no need to hold additional reference
to device kobject in provider driver.

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