scsi: lpfc: Fix unused assignment in lpfc_sli4_bsg_link_diag_test

Coverity reported the following error:

  Assigned value that is never used may represent unnecessary computation.

The rc variable was initially assigned a value but in several cases, when
an error case is detected, it is reassigned a new value. The initial value
had little use.

In code-reviewing this routine, it could use some cleanup:

 - Setting the initialization value to -ENODEV is a much better choice and
   lessens code in the routine.

 - The wasn't tracking logic errors vs no error and mailbox failure.
   Better to resolve by adding a status to track the mailbox failure
   and merge it with the logic error when the routine returns.

Link: https://lore.kernel.org/r/20200630215001.70793-2-jsmart2021@gmail.com


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>