RDMA/hw/qib/qib_tx: Provide description for 'qib_chg_pioavailkernel()'s 'rcd' param

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

 drivers/infiniband/hw/qib/qib_tx.c:383: warning: Function parameter or member 'rcd' not described in 'qib_chg_pioavailkernel'

Link: https://lore.kernel.org/r/20210121094519.2044049-18-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_tx.c

@@ -377,6 +377,7 @@ void qib_sendbuf_done(struct qib_devdata *dd, unsigned n)
  * @start: the starting send buffer number
  * @len: the number of send buffers
  * @avail: true if the buffers are available for kernel use, false otherwise
+ * @rcd: the context pointer
  */
 void qib_chg_pioavailkernel(struct qib_devdata *dd, unsigned start,
 	unsigned len, u32 avail, struct qib_ctxtdata *rcd)