iscsi/iser-target: fix build warning

iscsi/iser-target: fix build warning

With upstream commit defd884845297fd5690594bfe89656b01f16d87e, 3.12
commit 2f82fa987df6b060241fc760eadf5dd02c51a979, we introduced a
warning:
  drivers/infiniband/ulp/isert/ib_isert.c: In function 'isert_cq_rx_comp_err':
  drivers/infiniband/ulp/isert/ib_isert.c:##L##: warning: unused variable 'ib_dev' [-Wunused-variable]

Remove ib_dev as it is unused.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: Or Gerlitz <ogerlitz@mellanox.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>