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>

drivers/infiniband/hw/qib/qib_twsi.c

@@ -168,6 +168,7 @@ static void stop_cmd(struct qib_devdata *dd);
 /**
  * rd_byte - read a byte, sending STOP on last, else ACK
  * @dd: the qlogic_ib device
+ * @last: identifies the last read
  *
  * Returns byte shifted out of device
  */