[SCSI] ibmvfc: Fix errors due to inconsistent command data

In order to ensure the VIOS sees a consistent command buffer, we
need to add a memory barrier after building the command buffer
but before sending the command.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>