RDMA/nldev: Enforce device index check for port callback

IB device index is nldev's handler and it should be checked always.

Fixes: c3f66f7b

 ("RDMA/netlink: Implement nldev port doit callback")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Acked-by: Doug Ledford <dledford@redhat.com>
[ Applying directly, since Doug fried his SSD's and is rebuilding  - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>