scsi: lpfc: Resolve checker warning for lpfc_new_io_buf()

Per Dan Carpenter:

The patch d79c9e9d: "scsi: lpfc: Support dynamic unbounded SGL lists on
G7 hardware." from Aug 14, 2019, leads to the following static checker
warning:

   drivers/scsi/lpfc/lpfc_init.c:4107 lpfc_new_io_buf()
  error: not allocating enough data 784 vs 768

There was no need to compare sizes nor to allocate size based on a define.

Change allocation to use actual structure length

Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <jsmart2021@gmail.com>
CC: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>