target/iscsi: Remove unneeded locking from iscsi_target_tx_thread

When processing immediate queue, we're switching on a local variable
so it's not necessary to lock around it. However, we are modifying
cmd->i_state in two spots, so lock around those parts only.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>