RDMA/ipoib: Return void from ipoib_mcast_stop_thread()

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

Link: https://lore.kernel.org/r/20200622092256.6931-1-kamalheib1@gmail.com
Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>