[SCSI] qla4xxx: correctly update session discovery_parent_idx.

Earlier logic for driver created iscsi_session->discovery_parent_idx
was to store ram index of a sendtarget entry, but driver frees
sendtarget ram index as soon as firmware is done with discovery,
which is available for further use. So changing the logic to point
iscsi_session->discovery_parent_idx to store sendtarget flashnode index.

Signed-off-by: Manish Rangankar <manish.rangankar@qlogic.com>
Signed-off-by: Vikas Chaudhary <vikas.chaudhary@qlogic.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>