IB/umad: Start using dev_groups of class

Start using core defined dev_groups of a class which allows to add device
attributes to the core kernel and simplify the umad module.

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