scsi: ibmvscsi: add write memory barrier to CRQ processing

The first byte of each CRQ entry is used to indicate whether an entry is
a valid response or free for the VIOS to use. After processing a
response the driver sets the valid byte to zero to indicate the entry is
now free to be reused. Add a memory barrier after this write to ensure
no other stores are reordered when updating the valid byte.

Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Reviewed-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>