scsi: hpsa: update discovery polling

Correct a corner case where newly created volumes are not detected
automatically on an external RAID controller that has no configured
volumes during initial device discovery.

The fix is to set the discovery_polling flag when an external RAID
controller is detected. This causes a device rescan every 20-30 seconds,
so that newly created volumes will be detected automatically.

Reviewed-by: Scott Benesh <scott.benesh@microsemi.com>
Reviewed-by: Scott Teel <scott.teel@microsemi.com>
Reviewed-by: Kevin Barnett <kevin.barnett@microsemi.com>
Signed-off-by: Don Brace <don.brace@microsemi.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>