RDMA/cma: Simplify netdev check

Current code checks for NULL ndev twice where 2nd check is always
invalid given the fact that during route resolving stage, device address
must be bound to netdevice interface.

This patch simplifies such check.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>