RDMA/hw/qib/qib_twsi: Provide description for missing param 'last'

RDMA/hw/qib/qib_twsi: Provide description for missing param 'last'

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

 drivers/infiniband/hw/qib/qib_twsi.c:175: warning: Function parameter or member 'last' not described in 'rd_byte'

Link: https://lore.kernel.org/r/20210121094519.2044049-17-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>