RDMA/nldev: Refactor setting the nldev handle to a common function

The NLDEV commands are using IB device indexes and names as a handle
for netlink communications. Put all relevant code into one function
to remove code duplication in followup patches.

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