[SCSI] sas: clear parent->rphy in sas_rphy_delete

We need to clear the backpointer on rphy removal, else we'll run into
problems with host removal after a device has been hot unplugged.

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