scsi: qedf: Correct the memory barriers in qedf_ring_doorbell

 - Correct memory barriers to make sure all cmnds are flushed.

Signed-off-by: Andrew Vasquez <andrewv@marvell.com>
Signed-off-by: Saurav Kashyap <skashyap@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>