RDMA/sw/rdmavt/mcast: Demote incomplete kernel-doc header

RDMA/sw/rdmavt/mcast: Demote incomplete kernel-doc header

Fixes the following W=1 kernel build warning(s):

 drivers/infiniband/sw/rdmavt/mcast.c:195: warning: Function parameter or member 'rdi' not described in 'rvt_mcast_add'
 drivers/infiniband/sw/rdmavt/mcast.c:195: warning: Function parameter or member 'ibp' not described in 'rvt_mcast_add'

Link: https://lore.kernel.org/r/20210121094519.2044049-23-lee.jones@linaro.org
Cc: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Cc: Mike Marciniszyn <mike.marciniszyn@cornelisnetworks.com>
Cc: Doug Ledford <dledford@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>