iser-target: Move INIT_WORK setup into isert_create_device_ib_res

This patch moves INIT_WORK setup for cq_desc->cq_[rx,tx]_work into
isert_create_device_ib_res(), instead of being done each callback
invocation in isert_cq_[rx,tx]_callback().

This also fixes a 'INFO: trying to register non-static key' warning
when cancel_work_sync() is called before INIT_WORK has setup the
struct work_struct.

Reported-by: Or Gerlitz <ogerlitz@mellanox.com>
Cc: <stable@vger.kernel.org> #3.12+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>