target: Convert se_cmd->t_bidi_data_sg checks to use SCF_BIDI

Stop keying off se_cmd->t_bidi_data_sg within transport_complete_qf()
+ target_complete_ok_work(), and just use SCF_BIDI instead.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Martin Petersen <martin.petersen@oracle.com>
Cc: Chris Mason <chris.mason@fusionio.com>
Cc: James Bottomley <JBottomley@Parallels.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Nicholas Bellinger <nab@daterainc.com>