target: Change default sense key of NOT_READY

As the comment sais, this allows Solaris initiators to survive
intermittent errors.  The comment from someone reading the Solaris
sources seem to imply that multipathing would be broken without this
patch as well.

Signed-off-by: Joern Engel <joern@logfs.org>
Cc: Brian Bunker <brian@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>