scsi: qla2xxx: Fix buffer-buffer credit extraction error

Current code uses wrong mailbox option to extract bbc from firmware. This
field is nested inside of PLOGI payload.  Extract bbc from PLOGI template
payload.

Link: https://lore.kernel.org/r/20200929102152.32278-3-njavali@marvell.com


Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Quinn Tran <qutran@marvell.com>
Signed-off-by: Nilesh Javali <njavali@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>