[PATCH] libata-hp: implement SCSI part of hotplug

Implement SCSI part of hotplug.

This must be done in a separate context as SCSI makes use of EH during
probing.  SCSI scan fails silently if EH is in progress.  In such
cases, libata pauses briefly and retries until every device is
attached.

Signed-off-by: Tejun Heo <htejun@gmail.com>