Instead of doing two step process to add char device and create underlying
device, use cdev_device_add() which does both.

Currently a kobject per uverbs_device is created to keep reference to its
holding ib_uverbs_device in addition to its underlying device 'dev'.

Instead just use uverbs_device->dev to keep a reference to.

With this change there is single reference tracker for ib_uverbs_device
structure.

This allows for subsequent patch to registers group attribute as well
using single API cdev_device_add().
Signed-off-by: NParav Pandit <parav@mellanox.com>
Reviewed-by: NDaniel Jurgens <danielj@mellanox.com>
Signed-off-by: NLeon Romanovsky <leonro@mellanox.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>