IB/core: Fix unused variable warning

Fix the below "make W=1" build warning:

    drivers/infiniband/core/iwcm.c: In function ‘destroy_cm_id’:
    drivers/infiniband/core/iwcm.c:330: warning: variable ‘ret’ set but not used

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>