RDMA/core: Replace open-coded variant of get_device

Reuse existing get_device() API to do it symmetric to already used
put_device() in commit 924b8900 ("RDMA/core: Replace open-coded
variant of put_device")

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