RDMA/qedr: Fix NULL pointer dereference when running over iWARP without RDMA-CM

Some RoCE specific code in qedr_modify_qp was run over an iWARP device
when running perftest benchmarks without the -R option.

The commit 3e44e0ee ("IB/providers: Avoid null netdev check for RoCE")
exposed this. Dropping the check for NULL pointer on ndev in
qedr_modify_qp lead to a null pointer dereference when running over
iWARP. Before the code would identify ndev as being NULL and return an
error.

Fixes: 3e44e0ee

 ("IB/providers: Avoid null netdev check for RoCE")
Signed-off-by: Ariel Elior <Ariel.Elior@cavium.com>
Signed-off-by: Michal Kalderon <Michal.Kalderon@cavium.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>