scsi: lpfc: Add MDS driver loopback diagnostics support

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>