IB/providers: Avoid null netdev check for RoCE

Now that IB core GID cache ensures that all RoCE entries have an
associated netdev remove null checks from the provider drivers for
clarity.

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