iser-target: Fix wc->wr_id cast warning

CC [M]  drivers/infiniband/ulp/isert/ib_isert.o
drivers/infiniband/ulp/isert/ib_isert.c: In function ‘isert_cq_comp_err’:
drivers/infiniband/ulp/isert/ib_isert.c:1979:42: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>