RDMA/ipoib: Return void from ipoib_ib_dev_stop()

The return value from ipoib_ib_dev_stop() is always 0 - change it to be
void.

Link: https://lore.kernel.org/r/20200623105236.18683-1-kamalheib1@gmail.com
Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>