[SCSI] qla2xxx: Reject loopback request if one is already in progress.

If another function on the port has initiated a loopback operation do not
process the current request.

Signed-off-by: Chad Dupuis <chad.dupuis@qlogic.com>
Signed-off-by: Saurav Kashyap <saurav.kashyap@qlogic.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>