RDMA/ipoib: Fix use of sizeof()

Make sure to use sizeof(...) instead of sizeof ... which is more
preferred.

Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>