RDMA/drivers: Use core provided API for registering device attributes

Use rdma_set_device_sysfs_group() to register device attributes and
simplify the driver.

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