RDMA/core: Replace open-coded variant of put_device

There is an existing function to decrease reference counter
of the device, let's use it.

Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>