[SCSI] sas: fix removal of devices behind expanders

We need to iterate over all children when removing and expander, else
stale objects will be around after host removal.  This fixes the oops
Eric Moore saw when removing and reloading mptsas.

Also don't try the scsi_remove_target call unless operating on an end
device.  The current unconditional call is harmless but confusing.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>