iser-target: Fix identification of login rx descriptor type

Once connection request is accepted, one rx descriptor
is posted to receive login request. This descriptor has rx type,
but is outside the main pool of rx descriptors, and thus
was mistreated as tx type.

Signed-off-by: Jenny Derzhavetz <jennyf@mellanox.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Cc: stable@vger.kernel.org # v3.10+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>