scsi: lpfc: Add loopback testing to trunking mode

When in trunking mode, the adapter can be placed into diagnostic mode and
each link in the trunk tested via loopback.

Add support to the driver to perform per-link loopback testing when in
trunking mode.

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>