Implement bootplug - boot probing via hotplug path.  While loading,
ata_host_add() simply schedules probing and invokes EH.  After EH
completes, ata_host_add() scans and assicates them with SCSI devices.
EH path is slightly modified to handle this (e.g. no autopsy during
bootplug).  The SCSI part is left in ata_host_add() because it's
shared with legacy path and to keep probing order as before (ATA scan
all ports in host_set then attach all).
Signed-off-by: Tejun Heo <htejun@gmail.com>