Added code to support driver loopback with MDS Diagnostics.  This style of
diagnostics passes frames from the fabric to the driver who then echo them
back out the link.  SEND_FRAME WQEs are used to transmit the frames.  Added
the SOF and EOF field location definitions for use by SEND_FRAME.

Also ensure that enable_mds_diags is a RW parameter.
Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>