target/iscsi: Simplify iscsit_free_cmd()

Since .se_tfo is only set if a command has been submitted to
the LIO core, check .se_tfo instead of .iscsi_opcode. Since
__iscsit_free_cmd() only affects SCSI commands but not TMFs,
calling that function for TMFs does not change behavior. This
patch does not change the behavior of iscsit_free_cmd().

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andy Grover <agrover@redhat.com>
Cc: David Disseldorp <ddiss@suse.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>