target: Fix parameter list length checking in MODE SELECT

An empty parameter list (length == 0) is not an error, so succeed MODE
SELECT in this case.  If the parameter list length is too small,
return the correct sense code of PARAMETER LIST LENGTH ERROR.

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