[SCSI] ibmvfc: Improve ibmvfc_async_desc memory layout

By changing field ordering we can avoid a couple of memory holes in
the tables that use the ibmvfc_async_desc structure.

Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
Acked-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>