[SCSI] scsi_dh_alua: fix submit_stpg return

submit_stpg() will always return failure so alua_activate() will report
failure via dm-multipath callback function.  Even though the stpg fired
successfuly dm-multipath does not know and always fails to change the
valid path.

By returning SCSI_DH_OK we're now skipping alua_activate()'s call to
activate_complete 'fn'.  But this is fine because stpg_endio() will call
it via h->callback_fn().

Signed-off-by: Joseph Gruher <joseph.r.gruher@intel.com>
Signed-off-by: Ilgu Hong <ilgu.hong@promise.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>