IB/umad: Simplify and avoid dynamic allocation of class

Simplify code to have a static structure instance for umad class
allocation.

This will allow to have class attributes defined along with class
registration in subsequent patch and allows more class methods definition
similar to ib_core 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>