xprtrdma: Update comments in frwr_op_send

Commit f2877623 ("xprtrdma: Chain Send to FastReg WRs") was
written before commit ce5b3717 ("xprtrdma: Replace all usage of
"frmr" with "frwr""), but was merged afterwards. Thus it still
refers to FRMR and MWs.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>