scsi_scan: don't dump trace when scsi_prep_async_scan() is called twice

The only user of scsi_prep_async_scan() is scsi_scan_host() and it
handles the situation correctly. Move 'called twice' reporting to debug
level as well.

The issue is observed on Hyper-V: on any device add/remove event storvsc
driver calls scsi_scan_host() and in case previous scan is still running
we get the message and stack dump on console.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Tested-by: Alex Ng <alexng@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>