target: Simplify command completion by removing CMD_T_FAILED flag

The CMD_T_FAILED flag is set used in one place to record the result of a
trivial test, and it is only tested once, few lines later.  We might as
well make the code simpler and easier to read by directly doing the test
of "success" where we want to use it.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>