RDMA: Convert destroy_wq to be void

All callers of destroy WQ are always success and there is no need
to check their return value, so convert destroy_wq to be void.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>